Commit ea693aaa authored by Hugh Dickins's avatar Hugh Dickins Committed by Andrew Morton
Browse files

mm/shmem: hold shmem_swaplist spinlock (not mutex) much less 

A flamegraph (from an MGLRU load) showed shmem_writeout()'s use of the
global shmem_swaplist_mutex worryingly hot: improvement is long overdue.

3.1 commit 6922c0c7 ("tmpfs: convert shmem_writepage and enable swap")
apologized for extending shmem_swaplist_mutex across add_to_swap_cache(),
and hoped to find another way: yes, there may be lots of work to allocate
radix tree nodes in there.  Then 6.15 commit b487a2da ("mm, swap:
simplify folio swap allocation") will have made it worse, by moving
shmem_writeout()'s swap allocation under that mutex too (but the worrying
flamegraph was observed even before that change).

There's a useful comment about pagelock no longer protecting from eviction
once moved to swap cache: but it's good till
shmem_delete_from_page_cache() replaces page pointer by swap entry, so
move the swaplist add between them.

We would much prefer to take the global lock once per inode than once per
page: given the possible races with shmem_unuse() pruning when !swapped
(and other tasks racing to swap other pages out or in), try the swaplist
add whenever swapped was incremented from 0 (but inode may already be on
the list - only unuse and evict bother to remove it).

This technique is more subtle than it looks (we're avoiding the very lock
which would make it easy), but works: whereas an unlocked list_empty()
check runs a risk of the inode being unqueued and left off the swaplist
forever, swapoff only completing when the page is faulted in or removed.

The need for a sleepable mutex went away in 5.1 commit b56a2d8a ("mm:
rid swapoff of quadratic complexity"): a spinlock works better now.

This commit is certain to take shmem_swaplist_mutex out of contention, and
has been seen to make a practical improvement (but there is likely to have
been an underlying issue which made its contention so visible).

Link: https://lkml.kernel.org/r/87beaec6-a3b0-ce7a-c892-1e1e5bd57aa3@google.com


Signed-off-by: default avatarHugh Dickins <hughd@google.com>
Reviewed-by: default avatarBaolin Wang <baolin.wang@linux.alibaba.com>
Tested-by: default avatarBaolin Wang <baolin.wang@linux.alibaba.com>
Tested-by: default avatarDavid Rientjes <rientjes@google.com>
Reviewed-by: default avatarKairui Song <kasong@tencent.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Barry Song <21cnbao@gmail.com>
Cc: Chris Li <chrisl@kernel.org>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent d53f2482

mm/shmem.c

+33 −26
Original line number Diff line number Diff line
@@ -292,7 +292,7 @@ bool vma_is_shmem(struct vm_area_struct *vma) 
}



static LIST_HEAD(shmem_swaplist);

static DEFINE_MUTEX(shmem_swaplist_mutex);

static DEFINE_SPINLOCK(shmem_swaplist_lock);



#ifdef CONFIG_TMPFS_QUOTA
@@ -432,10 +432,13 @@ static void shmem_free_inode(struct super_block *sb, size_t freed_ispace) 
 *

 * But normally   info->alloced == inode->i_mapping->nrpages + info->swapped

 * So mm freed is info->alloced - (inode->i_mapping->nrpages + info->swapped)

 *

 * Return: true if swapped was incremented from 0, for shmem_writeout().

 */

static void shmem_recalc_inode(struct inode *inode, long alloced, long swapped)

static bool shmem_recalc_inode(struct inode *inode, long alloced, long swapped)

{

	struct shmem_inode_info *info = SHMEM_I(inode);

	bool first_swapped = false;

	long freed;



	spin_lock(&info->lock);
@@ -450,8 +453,11 @@ static void shmem_recalc_inode(struct inode *inode, long alloced, long swapped) 
	 * to stop a racing shmem_recalc_inode() from thinking that a page has

	 * been freed.  Compensate here, to avoid the need for a followup call.

	 */

	if (swapped > 0)

	if (swapped > 0) {

		if (info->swapped == swapped)

			first_swapped = true;

		freed += swapped;

	}

	if (freed > 0)

		info->alloced -= freed;

	spin_unlock(&info->lock);
@@ -459,6 +465,7 @@ static void shmem_recalc_inode(struct inode *inode, long alloced, long swapped) 
	/* The quota case may block */

	if (freed > 0)

		shmem_inode_unacct_blocks(inode, freed);

	return first_swapped;

}



bool shmem_charge(struct inode *inode, long pages)
@@ -1375,11 +1382,11 @@ static void shmem_evict_inode(struct inode *inode) 
			/* Wait while shmem_unuse() is scanning this inode... */

			wait_var_event(&info->stop_eviction,

				       !atomic_read(&info->stop_eviction));

			mutex_lock(&shmem_swaplist_mutex);

			spin_lock(&shmem_swaplist_lock);

			/* ...but beware of the race if we peeked too early */

			if (!atomic_read(&info->stop_eviction))

				list_del_init(&info->swaplist);

			mutex_unlock(&shmem_swaplist_mutex);

			spin_unlock(&shmem_swaplist_lock);

		}

	}
@@ -1502,7 +1509,7 @@ int shmem_unuse(unsigned int type) 
	if (list_empty(&shmem_swaplist))

		return 0;



	mutex_lock(&shmem_swaplist_mutex);

	spin_lock(&shmem_swaplist_lock);

start_over:

	list_for_each_entry_safe(info, next, &shmem_swaplist, swaplist) {

		if (!info->swapped) {
@@ -1516,12 +1523,12 @@ int shmem_unuse(unsigned int type) 
		 * (igrab() would protect from unlink, but not from unmount).

		 */

		atomic_inc(&info->stop_eviction);

		mutex_unlock(&shmem_swaplist_mutex);

		spin_unlock(&shmem_swaplist_lock);



		error = shmem_unuse_inode(&info->vfs_inode, type);

		cond_resched();



		mutex_lock(&shmem_swaplist_mutex);

		spin_lock(&shmem_swaplist_lock);

		if (atomic_dec_and_test(&info->stop_eviction))

			wake_up_var(&info->stop_eviction);

		if (error)
@@ -1532,7 +1539,7 @@ int shmem_unuse(unsigned int type) 
		if (!info->swapped)

			list_del_init(&info->swaplist);

	}

	mutex_unlock(&shmem_swaplist_mutex);

	spin_unlock(&shmem_swaplist_lock);



	return error;

}
@@ -1622,30 +1629,30 @@ int shmem_writeout(struct folio *folio, struct swap_iocb **plug, 
		folio_mark_uptodate(folio);

	}



	if (!folio_alloc_swap(folio, __GFP_HIGH | __GFP_NOMEMALLOC | __GFP_NOWARN)) {

		bool first_swapped = shmem_recalc_inode(inode, 0, nr_pages);



		/*

		 * Add inode to shmem_unuse()'s list of swapped-out inodes,

		 * if it's not already there.  Do it now before the folio is

	 * moved to swap cache, when its pagelock no longer protects

	 * the inode from eviction.  But don't unlock the mutex until

	 * we've incremented swapped, because shmem_unuse_inode() will

	 * prune a !swapped inode from the swaplist under this mutex.

		 * removed from page cache, when its pagelock no longer

		 * protects the inode from eviction.  And do it now, after

		 * we've incremented swapped, because shmem_unuse() will

		 * prune a !swapped inode from the swaplist.

		 */

	mutex_lock(&shmem_swaplist_mutex);

		if (first_swapped) {

			spin_lock(&shmem_swaplist_lock);

			if (list_empty(&info->swaplist))

				list_add(&info->swaplist, &shmem_swaplist);

			spin_unlock(&shmem_swaplist_lock);

		}



	if (!folio_alloc_swap(folio, __GFP_HIGH | __GFP_NOMEMALLOC | __GFP_NOWARN)) {

		shmem_recalc_inode(inode, 0, nr_pages);

		swap_shmem_alloc(folio->swap, nr_pages);

		shmem_delete_from_page_cache(folio, swp_to_radix_entry(folio->swap));



		mutex_unlock(&shmem_swaplist_mutex);

		BUG_ON(folio_mapped(folio));

		return swap_writeout(folio, plug);

	}

	if (!info->swapped)

		list_del_init(&info->swaplist);

	mutex_unlock(&shmem_swaplist_mutex);

	if (nr_pages > 1)

		goto try_split;

redirty: