fs/erofs/zutil.c - linux - Git at Google

 // SPDX-License-Identifier: GPL-2.0-only
 /*
  * Copyright (C) 2018 HUAWEI, Inc.
  *             https://www.huawei.com/
  */
 #include "internal.h"

 struct z_erofs_gbuf {
 	spinlock_t lock;
 	void *ptr;
 	struct page **pages;
 	unsigned int nrpages;
 };

 static struct z_erofs_gbuf *z_erofs_gbufpool, *z_erofs_rsvbuf;
 static unsigned int z_erofs_gbuf_count, z_erofs_gbuf_nrpages,
 		z_erofs_rsv_nrpages;

 module_param_named(global_buffers, z_erofs_gbuf_count, uint, 0444);
 module_param_named(reserved_pages, z_erofs_rsv_nrpages, uint, 0444);

 static atomic_long_t erofs_global_shrink_cnt;	/* for all mounted instances */
 /* protected by 'erofs_sb_list_lock' */
 static unsigned int shrinker_run_no;

 /* protects the mounted 'erofs_sb_list' */
 static DEFINE_SPINLOCK(erofs_sb_list_lock);
 static LIST_HEAD(erofs_sb_list);
 static struct shrinker *erofs_shrinker_info;

 static unsigned int z_erofs_gbuf_id(void)
 {
 	return raw_smp_processor_id() % z_erofs_gbuf_count;
 }

 void *z_erofs_get_gbuf(unsigned int requiredpages)
 	__acquires(gbuf->lock)
 {
 	struct z_erofs_gbuf *gbuf;

 	migrate_disable();
 	gbuf = &z_erofs_gbufpool[z_erofs_gbuf_id()];
 	spin_lock(&gbuf->lock);
 	/* check if the buffer is too small */
 	if (requiredpages > gbuf->nrpages) {
 		spin_unlock(&gbuf->lock);
 		migrate_enable();
 		/* (for sparse checker) pretend gbuf->lock is still taken */
 		__acquire(gbuf->lock);
 		return NULL;
 	}
 	return gbuf->ptr;
 }

 void z_erofs_put_gbuf(void *ptr) __releases(gbuf->lock)
 {
 	struct z_erofs_gbuf *gbuf;

 	gbuf = &z_erofs_gbufpool[z_erofs_gbuf_id()];
 	DBG_BUGON(gbuf->ptr != ptr);
 	spin_unlock(&gbuf->lock);
 	migrate_enable();
 }

 int z_erofs_gbuf_growsize(unsigned int nrpages)
 {
 	static DEFINE_MUTEX(gbuf_resize_mutex);
 	struct page **tmp_pages = NULL;
 	struct z_erofs_gbuf *gbuf;
 	void *ptr, *old_ptr;
 	int last, i, j;

 	mutex_lock(&gbuf_resize_mutex);
 	/* avoid shrinking gbufs, since no idea how many fses rely on */
 	if (nrpages <= z_erofs_gbuf_nrpages) {
 		mutex_unlock(&gbuf_resize_mutex);
 		return 0;
 	}

 	for (i = 0; i < z_erofs_gbuf_count; ++i) {
 		gbuf = &z_erofs_gbufpool[i];
 		tmp_pages = kcalloc(nrpages, sizeof(*tmp_pages), GFP_KERNEL);
 		if (!tmp_pages)
 			goto out;

 		for (j = 0; j < gbuf->nrpages; ++j)
 			tmp_pages[j] = gbuf->pages[j];
 		do {
 			last = j;
 			j = alloc_pages_bulk_array(GFP_KERNEL, nrpages,
 						   tmp_pages);
 			if (last == j)
 				goto out;
 		} while (j != nrpages);

 		ptr = vmap(tmp_pages, nrpages, VM_MAP, PAGE_KERNEL);
 		if (!ptr)
 			goto out;

 		spin_lock(&gbuf->lock);
 		kfree(gbuf->pages);
 		gbuf->pages = tmp_pages;
 		old_ptr = gbuf->ptr;
 		gbuf->ptr = ptr;
 		gbuf->nrpages = nrpages;
 		spin_unlock(&gbuf->lock);
 		if (old_ptr)
 			vunmap(old_ptr);
 	}
 	z_erofs_gbuf_nrpages = nrpages;
 out:
 	if (i < z_erofs_gbuf_count && tmp_pages) {
 		for (j = 0; j < nrpages; ++j)
 			if (tmp_pages[j] && (j >= gbuf->nrpages ||
 					     tmp_pages[j] != gbuf->pages[j]))
 				__free_page(tmp_pages[j]);
 		kfree(tmp_pages);
 	}
 	mutex_unlock(&gbuf_resize_mutex);
 	return i < z_erofs_gbuf_count ? -ENOMEM : 0;
 }

 int __init z_erofs_gbuf_init(void)
 {
 	unsigned int i, total = num_possible_cpus();

 	if (z_erofs_gbuf_count)
 		total = min(z_erofs_gbuf_count, total);
 	z_erofs_gbuf_count = total;

 	/* The last (special) global buffer is the reserved buffer */
 	total += !!z_erofs_rsv_nrpages;

 	z_erofs_gbufpool = kcalloc(total, sizeof(*z_erofs_gbufpool),
 				   GFP_KERNEL);
 	if (!z_erofs_gbufpool)
 		return -ENOMEM;

 	if (z_erofs_rsv_nrpages) {
 		z_erofs_rsvbuf = &z_erofs_gbufpool[total - 1];
 		z_erofs_rsvbuf->pages = kcalloc(z_erofs_rsv_nrpages,
 				sizeof(*z_erofs_rsvbuf->pages), GFP_KERNEL);
 		if (!z_erofs_rsvbuf->pages) {
 			z_erofs_rsvbuf = NULL;
 			z_erofs_rsv_nrpages = 0;
 		}
 	}
 	for (i = 0; i < total; ++i)
 		spin_lock_init(&z_erofs_gbufpool[i].lock);
 	return 0;
 }

 void z_erofs_gbuf_exit(void)
 {
 	int i, j;

 	for (i = 0; i < z_erofs_gbuf_count + (!!z_erofs_rsvbuf); ++i) {
 		struct z_erofs_gbuf *gbuf = &z_erofs_gbufpool[i];

 		if (gbuf->ptr) {
 			vunmap(gbuf->ptr);
 			gbuf->ptr = NULL;
 		}

 		if (!gbuf->pages)
 			continue;

 		for (j = 0; j < gbuf->nrpages; ++j)
 			if (gbuf->pages[j])
 				put_page(gbuf->pages[j]);
 		kfree(gbuf->pages);
 		gbuf->pages = NULL;
 	}
 	kfree(z_erofs_gbufpool);
 }

 struct page *__erofs_allocpage(struct page **pagepool, gfp_t gfp, bool tryrsv)
 {
 	struct page *page = *pagepool;

 	if (page) {
 		*pagepool = (struct page *)page_private(page);
 	} else if (tryrsv && z_erofs_rsvbuf && z_erofs_rsvbuf->nrpages) {
 		spin_lock(&z_erofs_rsvbuf->lock);
 		if (z_erofs_rsvbuf->nrpages)
 			page = z_erofs_rsvbuf->pages[--z_erofs_rsvbuf->nrpages];
 		spin_unlock(&z_erofs_rsvbuf->lock);
 	}
 	if (!page)
 		page = alloc_page(gfp);
 	DBG_BUGON(page && page_ref_count(page) != 1);
 	return page;
 }

 void erofs_release_pages(struct page **pagepool)
 {
 	while (*pagepool) {
 		struct page *page = *pagepool;

 		*pagepool = (struct page *)page_private(page);
 		/* try to fill reserved global pool first */
 		if (z_erofs_rsvbuf && z_erofs_rsvbuf->nrpages <
 				z_erofs_rsv_nrpages) {
 			spin_lock(&z_erofs_rsvbuf->lock);
 			if (z_erofs_rsvbuf->nrpages < z_erofs_rsv_nrpages) {
 				z_erofs_rsvbuf->pages[z_erofs_rsvbuf->nrpages++]
 						= page;
 				spin_unlock(&z_erofs_rsvbuf->lock);
 				continue;
 			}
 			spin_unlock(&z_erofs_rsvbuf->lock);
 		}
 		put_page(page);
 	}
 }

 static bool erofs_workgroup_get(struct erofs_workgroup *grp)
 {
 	if (lockref_get_not_zero(&grp->lockref))
 		return true;

 	spin_lock(&grp->lockref.lock);
 	if (__lockref_is_dead(&grp->lockref)) {
 		spin_unlock(&grp->lockref.lock);
 		return false;
 	}

 	if (!grp->lockref.count++)
 		atomic_long_dec(&erofs_global_shrink_cnt);
 	spin_unlock(&grp->lockref.lock);
 	return true;
 }

 struct erofs_workgroup *erofs_find_workgroup(struct super_block *sb,
 					     pgoff_t index)
 {
 	struct erofs_sb_info *sbi = EROFS_SB(sb);
 	struct erofs_workgroup *grp;

 repeat:
 	rcu_read_lock();
 	grp = xa_load(&sbi->managed_pslots, index);
 	if (grp) {
 		if (!erofs_workgroup_get(grp)) {
 			/* prefer to relax rcu read side */
 			rcu_read_unlock();
 			goto repeat;
 		}

 		DBG_BUGON(index != grp->index);
 	}
 	rcu_read_unlock();
 	return grp;
 }

 struct erofs_workgroup *erofs_insert_workgroup(struct super_block *sb,
 					       struct erofs_workgroup *grp)
 {
 	struct erofs_sb_info *const sbi = EROFS_SB(sb);
 	struct erofs_workgroup *pre;

 	DBG_BUGON(grp->lockref.count < 1);
 repeat:
 	xa_lock(&sbi->managed_pslots);
 	pre = __xa_cmpxchg(&sbi->managed_pslots, grp->index,
 			   NULL, grp, GFP_KERNEL);
 	if (pre) {
 		if (xa_is_err(pre)) {
 			pre = ERR_PTR(xa_err(pre));
 		} else if (!erofs_workgroup_get(pre)) {
 			/* try to legitimize the current in-tree one */
 			xa_unlock(&sbi->managed_pslots);
 			cond_resched();
 			goto repeat;
 		}
 		grp = pre;
 	}
 	xa_unlock(&sbi->managed_pslots);
 	return grp;
 }

 static void  __erofs_workgroup_free(struct erofs_workgroup *grp)
 {
 	atomic_long_dec(&erofs_global_shrink_cnt);
 	erofs_workgroup_free_rcu(grp);
 }

 void erofs_workgroup_put(struct erofs_workgroup *grp)
 {
 	if (lockref_put_or_lock(&grp->lockref))
 		return;

 	DBG_BUGON(__lockref_is_dead(&grp->lockref));
 	if (grp->lockref.count == 1)
 		atomic_long_inc(&erofs_global_shrink_cnt);
 	--grp->lockref.count;
 	spin_unlock(&grp->lockref.lock);
 }

 static bool erofs_try_to_release_workgroup(struct erofs_sb_info *sbi,
 					   struct erofs_workgroup *grp)
 {
 	int free = false;

 	spin_lock(&grp->lockref.lock);
 	if (grp->lockref.count)
 		goto out;

 	/*
 	 * Note that all cached pages should be detached before deleted from
 	 * the XArray. Otherwise some cached pages could be still attached to
 	 * the orphan old workgroup when the new one is available in the tree.
 	 */
 	if (erofs_try_to_free_all_cached_folios(sbi, grp))
 		goto out;

 	/*
 	 * It's impossible to fail after the workgroup is freezed,
 	 * however in order to avoid some race conditions, add a
 	 * DBG_BUGON to observe this in advance.
 	 */
 	DBG_BUGON(__xa_erase(&sbi->managed_pslots, grp->index) != grp);

 	lockref_mark_dead(&grp->lockref);
 	free = true;
 out:
 	spin_unlock(&grp->lockref.lock);
 	if (free)
 		__erofs_workgroup_free(grp);
 	return free;
 }

 static unsigned long erofs_shrink_workstation(struct erofs_sb_info *sbi,
 					      unsigned long nr_shrink)
 {
 	struct erofs_workgroup *grp;
 	unsigned int freed = 0;
 	unsigned long index;

 	xa_lock(&sbi->managed_pslots);
 	xa_for_each(&sbi->managed_pslots, index, grp) {
 		/* try to shrink each valid workgroup */
 		if (!erofs_try_to_release_workgroup(sbi, grp))
 			continue;
 		xa_unlock(&sbi->managed_pslots);

 		++freed;
 		if (!--nr_shrink)
 			return freed;
 		xa_lock(&sbi->managed_pslots);
 	}
 	xa_unlock(&sbi->managed_pslots);
 	return freed;
 }

 void erofs_shrinker_register(struct super_block *sb)
 {
 	struct erofs_sb_info *sbi = EROFS_SB(sb);

 	mutex_init(&sbi->umount_mutex);

 	spin_lock(&erofs_sb_list_lock);
 	list_add(&sbi->list, &erofs_sb_list);
 	spin_unlock(&erofs_sb_list_lock);
 }

 void erofs_shrinker_unregister(struct super_block *sb)
 {
 	struct erofs_sb_info *const sbi = EROFS_SB(sb);

 	mutex_lock(&sbi->umount_mutex);
 	/* clean up all remaining workgroups in memory */
 	erofs_shrink_workstation(sbi, ~0UL);

 	spin_lock(&erofs_sb_list_lock);
 	list_del(&sbi->list);
 	spin_unlock(&erofs_sb_list_lock);
 	mutex_unlock(&sbi->umount_mutex);
 }

 static unsigned long erofs_shrink_count(struct shrinker *shrink,
 					struct shrink_control *sc)
 {
 	return atomic_long_read(&erofs_global_shrink_cnt);
 }

 static unsigned long erofs_shrink_scan(struct shrinker *shrink,
 				       struct shrink_control *sc)
 {
 	struct erofs_sb_info *sbi;
 	struct list_head *p;

 	unsigned long nr = sc->nr_to_scan;
 	unsigned int run_no;
 	unsigned long freed = 0;

 	spin_lock(&erofs_sb_list_lock);
 	do {
 		run_no = ++shrinker_run_no;
 	} while (run_no == 0);

 	/* Iterate over all mounted superblocks and try to shrink them */
 	p = erofs_sb_list.next;
 	while (p != &erofs_sb_list) {
 		sbi = list_entry(p, struct erofs_sb_info, list);

 		/*
 		 * We move the ones we do to the end of the list, so we stop
 		 * when we see one we have already done.
 		 */
 		if (sbi->shrinker_run_no == run_no)
 			break;

 		if (!mutex_trylock(&sbi->umount_mutex)) {
 			p = p->next;
 			continue;
 		}

 		spin_unlock(&erofs_sb_list_lock);
 		sbi->shrinker_run_no = run_no;

 		freed += erofs_shrink_workstation(sbi, nr - freed);

 		spin_lock(&erofs_sb_list_lock);
 		/* Get the next list element before we move this one */
 		p = p->next;

 		/*
 		 * Move this one to the end of the list to provide some
 		 * fairness.
 		 */
 		list_move_tail(&sbi->list, &erofs_sb_list);
 		mutex_unlock(&sbi->umount_mutex);

 		if (freed >= nr)
 			break;
 	}
 	spin_unlock(&erofs_sb_list_lock);
 	return freed;
 }

 int __init erofs_init_shrinker(void)
 {
 	erofs_shrinker_info = shrinker_alloc(0, "erofs-shrinker");
 	if (!erofs_shrinker_info)
 		return -ENOMEM;

 	erofs_shrinker_info->count_objects = erofs_shrink_count;
 	erofs_shrinker_info->scan_objects = erofs_shrink_scan;
 	shrinker_register(erofs_shrinker_info);
 	return 0;
 }

 void erofs_exit_shrinker(void)
 {
 	shrinker_free(erofs_shrinker_info);
 }
	// SPDX-License-Identifier: GPL-2.0-only
	/*
	* Copyright (C) 2018 HUAWEI, Inc.
	* https://www.huawei.com/
	*/
	#include "internal.h"

	struct z_erofs_gbuf {
	spinlock_t lock;
	void *ptr;
	struct page **pages;
	unsigned int nrpages;
	};

	static struct z_erofs_gbuf z_erofs_gbufpool, z_erofs_rsvbuf;
	static unsigned int z_erofs_gbuf_count, z_erofs_gbuf_nrpages,
	z_erofs_rsv_nrpages;

	module_param_named(global_buffers, z_erofs_gbuf_count, uint, 0444);
	module_param_named(reserved_pages, z_erofs_rsv_nrpages, uint, 0444);

	static atomic_long_t erofs_global_shrink_cnt; /* for all mounted instances */
	/* protected by 'erofs_sb_list_lock' */
	static unsigned int shrinker_run_no;

	/* protects the mounted 'erofs_sb_list' */
	static DEFINE_SPINLOCK(erofs_sb_list_lock);
	static LIST_HEAD(erofs_sb_list);
	static struct shrinker *erofs_shrinker_info;

	static unsigned int z_erofs_gbuf_id(void)
	{
	return raw_smp_processor_id() % z_erofs_gbuf_count;
	}

	void *z_erofs_get_gbuf(unsigned int requiredpages)
	__acquires(gbuf->lock)
	{
	struct z_erofs_gbuf *gbuf;

	migrate_disable();
	gbuf = &z_erofs_gbufpool[z_erofs_gbuf_id()];
	spin_lock(&gbuf->lock);
	/* check if the buffer is too small */
	if (requiredpages > gbuf->nrpages) {
	spin_unlock(&gbuf->lock);
	migrate_enable();
	/* (for sparse checker) pretend gbuf->lock is still taken */
	__acquire(gbuf->lock);
	return NULL;
	}
	return gbuf->ptr;
	}

	void z_erofs_put_gbuf(void *ptr) __releases(gbuf->lock)
	{
	struct z_erofs_gbuf *gbuf;

	gbuf = &z_erofs_gbufpool[z_erofs_gbuf_id()];
	DBG_BUGON(gbuf->ptr != ptr);
	spin_unlock(&gbuf->lock);
	migrate_enable();
	}

	int z_erofs_gbuf_growsize(unsigned int nrpages)
	{
	static DEFINE_MUTEX(gbuf_resize_mutex);
	struct page **tmp_pages = NULL;
	struct z_erofs_gbuf *gbuf;
	void ptr, old_ptr;
	int last, i, j;

	mutex_lock(&gbuf_resize_mutex);
	/* avoid shrinking gbufs, since no idea how many fses rely on */
	if (nrpages <= z_erofs_gbuf_nrpages) {
	mutex_unlock(&gbuf_resize_mutex);
	return 0;
	}

	for (i = 0; i < z_erofs_gbuf_count; ++i) {
	gbuf = &z_erofs_gbufpool[i];
	tmp_pages = kcalloc(nrpages, sizeof(*tmp_pages), GFP_KERNEL);
	if (!tmp_pages)
	goto out;

	for (j = 0; j < gbuf->nrpages; ++j)
	tmp_pages[j] = gbuf->pages[j];
	do {
	last = j;
	j = alloc_pages_bulk_array(GFP_KERNEL, nrpages,
	tmp_pages);
	if (last == j)
	goto out;
	} while (j != nrpages);

	ptr = vmap(tmp_pages, nrpages, VM_MAP, PAGE_KERNEL);
	if (!ptr)
	goto out;

	spin_lock(&gbuf->lock);
	kfree(gbuf->pages);
	gbuf->pages = tmp_pages;
	old_ptr = gbuf->ptr;
	gbuf->ptr = ptr;
	gbuf->nrpages = nrpages;
	spin_unlock(&gbuf->lock);
	if (old_ptr)
	vunmap(old_ptr);
	}
	z_erofs_gbuf_nrpages = nrpages;
	out:
	if (i < z_erofs_gbuf_count && tmp_pages) {
	for (j = 0; j < nrpages; ++j)
	if (tmp_pages[j] && (j >= gbuf->nrpages \|\|
	tmp_pages[j] != gbuf->pages[j]))
	__free_page(tmp_pages[j]);
	kfree(tmp_pages);
	}
	mutex_unlock(&gbuf_resize_mutex);
	return i < z_erofs_gbuf_count ? -ENOMEM : 0;
	}

	int __init z_erofs_gbuf_init(void)
	{
	unsigned int i, total = num_possible_cpus();

	if (z_erofs_gbuf_count)
	total = min(z_erofs_gbuf_count, total);
	z_erofs_gbuf_count = total;

	/* The last (special) global buffer is the reserved buffer */
	total += !!z_erofs_rsv_nrpages;

	z_erofs_gbufpool = kcalloc(total, sizeof(*z_erofs_gbufpool),
	GFP_KERNEL);
	if (!z_erofs_gbufpool)
	return -ENOMEM;

	if (z_erofs_rsv_nrpages) {
	z_erofs_rsvbuf = &z_erofs_gbufpool[total - 1];
	z_erofs_rsvbuf->pages = kcalloc(z_erofs_rsv_nrpages,
	sizeof(*z_erofs_rsvbuf->pages), GFP_KERNEL);
	if (!z_erofs_rsvbuf->pages) {
	z_erofs_rsvbuf = NULL;
	z_erofs_rsv_nrpages = 0;
	}
	}
	for (i = 0; i < total; ++i)
	spin_lock_init(&z_erofs_gbufpool[i].lock);
	return 0;
	}

	void z_erofs_gbuf_exit(void)
	{
	int i, j;

	for (i = 0; i < z_erofs_gbuf_count + (!!z_erofs_rsvbuf); ++i) {
	struct z_erofs_gbuf *gbuf = &z_erofs_gbufpool[i];

	if (gbuf->ptr) {
	vunmap(gbuf->ptr);
	gbuf->ptr = NULL;
	}

	if (!gbuf->pages)
	continue;

	for (j = 0; j < gbuf->nrpages; ++j)
	if (gbuf->pages[j])
	put_page(gbuf->pages[j]);
	kfree(gbuf->pages);
	gbuf->pages = NULL;
	}
	kfree(z_erofs_gbufpool);
	}

	struct page __erofs_allocpage(struct page *pagepool, gfp_t gfp, bool tryrsv)
	{
	struct page page = pagepool;

	if (page) {
	pagepool = (struct page )page_private(page);
	} else if (tryrsv && z_erofs_rsvbuf && z_erofs_rsvbuf->nrpages) {
	spin_lock(&z_erofs_rsvbuf->lock);
	if (z_erofs_rsvbuf->nrpages)
	page = z_erofs_rsvbuf->pages[--z_erofs_rsvbuf->nrpages];
	spin_unlock(&z_erofs_rsvbuf->lock);
	}
	if (!page)
	page = alloc_page(gfp);
	DBG_BUGON(page && page_ref_count(page) != 1);
	return page;
	}

	void erofs_release_pages(struct page **pagepool)
	{
	while (*pagepool) {
	struct page page = pagepool;

	pagepool = (struct page )page_private(page);
	/* try to fill reserved global pool first */
	if (z_erofs_rsvbuf && z_erofs_rsvbuf->nrpages <
	z_erofs_rsv_nrpages) {
	spin_lock(&z_erofs_rsvbuf->lock);
	if (z_erofs_rsvbuf->nrpages < z_erofs_rsv_nrpages) {
	z_erofs_rsvbuf->pages[z_erofs_rsvbuf->nrpages++]
	= page;
	spin_unlock(&z_erofs_rsvbuf->lock);
	continue;
	}
	spin_unlock(&z_erofs_rsvbuf->lock);
	}
	put_page(page);
	}
	}

	static bool erofs_workgroup_get(struct erofs_workgroup *grp)
	{
	if (lockref_get_not_zero(&grp->lockref))
	return true;

	spin_lock(&grp->lockref.lock);
	if (__lockref_is_dead(&grp->lockref)) {
	spin_unlock(&grp->lockref.lock);
	return false;
	}

	if (!grp->lockref.count++)
	atomic_long_dec(&erofs_global_shrink_cnt);
	spin_unlock(&grp->lockref.lock);
	return true;
	}

	struct erofs_workgroup erofs_find_workgroup(struct super_block sb,
	pgoff_t index)
	{
	struct erofs_sb_info *sbi = EROFS_SB(sb);
	struct erofs_workgroup *grp;

	repeat:
	rcu_read_lock();
	grp = xa_load(&sbi->managed_pslots, index);
	if (grp) {
	if (!erofs_workgroup_get(grp)) {
	/* prefer to relax rcu read side */
	rcu_read_unlock();
	goto repeat;
	}

	DBG_BUGON(index != grp->index);
	}
	rcu_read_unlock();
	return grp;
	}

	struct erofs_workgroup erofs_insert_workgroup(struct super_block sb,
	struct erofs_workgroup *grp)
	{
	struct erofs_sb_info *const sbi = EROFS_SB(sb);
	struct erofs_workgroup *pre;

	DBG_BUGON(grp->lockref.count < 1);
	repeat:
	xa_lock(&sbi->managed_pslots);
	pre = __xa_cmpxchg(&sbi->managed_pslots, grp->index,
	NULL, grp, GFP_KERNEL);
	if (pre) {
	if (xa_is_err(pre)) {
	pre = ERR_PTR(xa_err(pre));
	} else if (!erofs_workgroup_get(pre)) {
	/* try to legitimize the current in-tree one */
	xa_unlock(&sbi->managed_pslots);
	cond_resched();
	goto repeat;
	}
	grp = pre;
	}
	xa_unlock(&sbi->managed_pslots);
	return grp;
	}

	static void __erofs_workgroup_free(struct erofs_workgroup *grp)
	{
	atomic_long_dec(&erofs_global_shrink_cnt);
	erofs_workgroup_free_rcu(grp);
	}

	void erofs_workgroup_put(struct erofs_workgroup *grp)
	{
	if (lockref_put_or_lock(&grp->lockref))
	return;

	DBG_BUGON(__lockref_is_dead(&grp->lockref));
	if (grp->lockref.count == 1)
	atomic_long_inc(&erofs_global_shrink_cnt);
	--grp->lockref.count;
	spin_unlock(&grp->lockref.lock);
	}

	static bool erofs_try_to_release_workgroup(struct erofs_sb_info *sbi,
	struct erofs_workgroup *grp)
	{
	int free = false;

	spin_lock(&grp->lockref.lock);
	if (grp->lockref.count)
	goto out;

	/*
	* Note that all cached pages should be detached before deleted from
	* the XArray. Otherwise some cached pages could be still attached to
	* the orphan old workgroup when the new one is available in the tree.
	*/
	if (erofs_try_to_free_all_cached_folios(sbi, grp))
	goto out;

	/*
	* It's impossible to fail after the workgroup is freezed,
	* however in order to avoid some race conditions, add a
	* DBG_BUGON to observe this in advance.
	*/
	DBG_BUGON(__xa_erase(&sbi->managed_pslots, grp->index) != grp);

	lockref_mark_dead(&grp->lockref);
	free = true;
	out:
	spin_unlock(&grp->lockref.lock);
	if (free)
	__erofs_workgroup_free(grp);
	return free;
	}

	static unsigned long erofs_shrink_workstation(struct erofs_sb_info *sbi,
	unsigned long nr_shrink)
	{
	struct erofs_workgroup *grp;
	unsigned int freed = 0;
	unsigned long index;

	xa_lock(&sbi->managed_pslots);
	xa_for_each(&sbi->managed_pslots, index, grp) {
	/* try to shrink each valid workgroup */
	if (!erofs_try_to_release_workgroup(sbi, grp))
	continue;
	xa_unlock(&sbi->managed_pslots);

	++freed;
	if (!--nr_shrink)
	return freed;
	xa_lock(&sbi->managed_pslots);
	}
	xa_unlock(&sbi->managed_pslots);
	return freed;
	}

	void erofs_shrinker_register(struct super_block *sb)
	{
	struct erofs_sb_info *sbi = EROFS_SB(sb);

	mutex_init(&sbi->umount_mutex);

	spin_lock(&erofs_sb_list_lock);
	list_add(&sbi->list, &erofs_sb_list);
	spin_unlock(&erofs_sb_list_lock);
	}

	void erofs_shrinker_unregister(struct super_block *sb)
	{
	struct erofs_sb_info *const sbi = EROFS_SB(sb);

	mutex_lock(&sbi->umount_mutex);
	/* clean up all remaining workgroups in memory */
	erofs_shrink_workstation(sbi, ~0UL);

	spin_lock(&erofs_sb_list_lock);
	list_del(&sbi->list);
	spin_unlock(&erofs_sb_list_lock);
	mutex_unlock(&sbi->umount_mutex);
	}

	static unsigned long erofs_shrink_count(struct shrinker *shrink,
	struct shrink_control *sc)
	{
	return atomic_long_read(&erofs_global_shrink_cnt);
	}

	static unsigned long erofs_shrink_scan(struct shrinker *shrink,
	struct shrink_control *sc)
	{
	struct erofs_sb_info *sbi;
	struct list_head *p;

	unsigned long nr = sc->nr_to_scan;
	unsigned int run_no;
	unsigned long freed = 0;

	spin_lock(&erofs_sb_list_lock);
	do {
	run_no = ++shrinker_run_no;
	} while (run_no == 0);

	/* Iterate over all mounted superblocks and try to shrink them */
	p = erofs_sb_list.next;
	while (p != &erofs_sb_list) {
	sbi = list_entry(p, struct erofs_sb_info, list);

	/*
	* We move the ones we do to the end of the list, so we stop
	* when we see one we have already done.
	*/
	if (sbi->shrinker_run_no == run_no)
	break;

	if (!mutex_trylock(&sbi->umount_mutex)) {
	p = p->next;
	continue;
	}

	spin_unlock(&erofs_sb_list_lock);
	sbi->shrinker_run_no = run_no;

	freed += erofs_shrink_workstation(sbi, nr - freed);

	spin_lock(&erofs_sb_list_lock);
	/* Get the next list element before we move this one */
	p = p->next;

	/*
	* Move this one to the end of the list to provide some
	* fairness.
	*/
	list_move_tail(&sbi->list, &erofs_sb_list);
	mutex_unlock(&sbi->umount_mutex);

	if (freed >= nr)
	break;
	}
	spin_unlock(&erofs_sb_list_lock);
	return freed;
	}

	int __init erofs_init_shrinker(void)
	{
	erofs_shrinker_info = shrinker_alloc(0, "erofs-shrinker");
	if (!erofs_shrinker_info)
	return -ENOMEM;

	erofs_shrinker_info->count_objects = erofs_shrink_count;
	erofs_shrinker_info->scan_objects = erofs_shrink_scan;
	shrinker_register(erofs_shrinker_info);
	return 0;
	}

	void erofs_exit_shrinker(void)
	{
	shrinker_free(erofs_shrinker_info);
	}