Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> wrote:
> Hi, Greg,
>
> good finding. See comments below.
>
> On 01.06.2020 06:22, Greg Thelen wrote:
>> Since v4.19 commit b0dedc49a2da ("mm/vmscan.c: iterate only over charged
>> shrinkers during memcg shrink_slab()") a memcg aware shrinker is only
>> called when the per-memcg per-node shrinker_map indicates that the
>> shrinker may have objects to release to the memcg and node.
>>
>> shmem_unused_huge_count and shmem_unused_huge_scan support the per-tmpfs
>> shrinker which advertises per memcg and numa awareness. The shmem
>> shrinker releases memory by splitting hugepages that extend beyond
>> i_size.
>>
>> Shmem does not currently set bits in shrinker_map. So, starting with
>> b0dedc49a2da, memcg reclaim avoids calling the shmem shrinker under
>> pressure. This leads to undeserved memcg OOM kills.
>> Example that reliably sees memcg OOM kill in unpatched kernel:
>> FS=/tmp/fs
>> CONTAINER=/cgroup/memory/tmpfs_shrinker
>> mkdir -p $FS
>> mount -t tmpfs -o huge=always nodev $FS
>> # Create 1000 MB container, which shouldn't suffer OOM.
>> mkdir $CONTAINER
>> echo 1000M > $CONTAINER/memory.limit_in_bytes
>> echo $BASHPID >> $CONTAINER/cgroup.procs
>> # Create 4000 files. Ideally each file uses 4k data page + a little
>> # metadata. Assume 8k total per-file, 32MB (4000*8k) should easily
>> # fit within container's 1000 MB. But if data pages use 2MB
>> # hugepages (due to aggressive huge=always) then files consume 8GB,
>> # which hits memcg 1000 MB limit.
>> for i in {1..4000}; do
>> echo . > $FS/$i
>> done
>>
>> v5.4 commit 87eaceb3faa5 ("mm: thp: make deferred split shrinker memcg
>> aware") maintains the per-node per-memcg shrinker bitmap for THP
>> shrinker. But there's no such logic in shmem. Make shmem set the
>> per-memcg per-node shrinker bits when it modifies inodes to have
>> shrinkable pages.
>>
>> Fixes: b0dedc49a2da ("mm/vmscan.c: iterate only over charged shrinkers during memcg shrink_slab()")
>> Cc: <stable@xxxxxxxxxxxxxxx> # 4.19+
>> Signed-off-by: Greg Thelen <gthelen@xxxxxxxxxx>
>> ---
>> mm/shmem.c | 61 +++++++++++++++++++++++++++++++-----------------------
>> 1 file changed, 35 insertions(+), 26 deletions(-)
>>
>> diff --git a/mm/shmem.c b/mm/shmem.c
>> index bd8840082c94..e11090f78cb5 100644
>> --- a/mm/shmem.c
>> +++ b/mm/shmem.c
>> @@ -1002,6 +1002,33 @@ static int shmem_getattr(const struct path *path, struct kstat *stat,
>> return 0;
>> }
>>
>> +/*
>> + * Expose inode and optional page to shrinker as having a possibly splittable
>> + * hugepage that reaches beyond i_size.
>> + */
>> +static void shmem_shrinker_add(struct shmem_sb_info *sbinfo,
>> + struct inode *inode, struct page *page)
>> +{
>> + struct shmem_inode_info *info = SHMEM_I(inode);
>> +
>> + spin_lock(&sbinfo->shrinklist_lock);
>> + /*
>> + * _careful to defend against unlocked access to ->shrink_list in
>> + * shmem_unused_huge_shrink()
>> + */
>> + if (list_empty_careful(&info->shrinklist)) {
>> + list_add_tail(&info->shrinklist, &sbinfo->shrinklist);
>> + sbinfo->shrinklist_len++;
>> + }
>> + spin_unlock(&sbinfo->shrinklist_lock);
>> +
>> +#ifdef CONFIG_MEMCG
>> + if (page && PageTransHuge(page))
>> + memcg_set_shrinker_bit(page->mem_cgroup, page_to_nid(page),
>> + inode->i_sb->s_shrink.id);
>> +#endif
>> +}
>> +
>> static int shmem_setattr(struct dentry *dentry, struct iattr *attr)
>> {
>> struct inode *inode = d_inode(dentry);
>> @@ -1048,17 +1075,13 @@ static int shmem_setattr(struct dentry *dentry, struct iattr *attr)
>> * to shrink under memory pressure.
>> */
>> if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) {
>> - spin_lock(&sbinfo->shrinklist_lock);
>> - /*
>> - * _careful to defend against unlocked access to
>> - * ->shrink_list in shmem_unused_huge_shrink()
>> - */
>> - if (list_empty_careful(&info->shrinklist)) {
>> - list_add_tail(&info->shrinklist,
>> - &sbinfo->shrinklist);
>> - sbinfo->shrinklist_len++;
>> - }
>> - spin_unlock(&sbinfo->shrinklist_lock);
>> + struct page *page;
>> +
>> + page = find_get_page(inode->i_mapping,
>> + (newsize & HPAGE_PMD_MASK) >> PAGE_SHIFT);
>> + shmem_shrinker_add(sbinfo, inode, page);
>> + if (page)
>> + put_page(page);
>
> 1)I'd move PageTransHuge() check from shmem_shrinker_add() to here. In case of page is not trans huge,
> it looks strange and completely useless to add inode to shrinklist and then to avoid memcg_set_shrinker_bit().
> Nothing should be added to the shrinklist in this case.
Ack, I'll take a look at this.
> 2)In general I think this "last inode page spliter" does not fit SHINKER_MEMCG_AWARE conception, and
> shmem_unused_huge_shrink() should be reworked as a new separate !memcg-aware shrinker instead of
> .nr_cached_objects callback of generic fs shrinker.
>
> CC: Kirill Shutemov
>
> Kirill, are there any fundamental reasons to keep this shrinking logic in the generic fs shrinker?
> Are there any no-go to for conversion this as a separate !memcg-aware shrinker?
Making the shmem shrinker !memcg-aware seems like it would require tail
pages beyond i_size not be charged to memcg. Otherwise memcg pressure
(which only calls memcg aware shrinkers) won't uncharge them. Currently
the entire compound page is charged.
>> }
>> }
>> }
>> @@ -1889,21 +1912,7 @@ static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
>> if (PageTransHuge(page) &&
>> DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE) <
>> hindex + HPAGE_PMD_NR - 1) {
>> - /*
>> - * Part of the huge page is beyond i_size: subject
>> - * to shrink under memory pressure.
>> - */
>> - spin_lock(&sbinfo->shrinklist_lock);
>> - /*
>> - * _careful to defend against unlocked access to
>> - * ->shrink_list in shmem_unused_huge_shrink()
>> - */
>> - if (list_empty_careful(&info->shrinklist)) {
>> - list_add_tail(&info->shrinklist,
>> - &sbinfo->shrinklist);
>> - sbinfo->shrinklist_len++;
>> - }
>> - spin_unlock(&sbinfo->shrinklist_lock);
>> + shmem_shrinker_add(sbinfo, inode, page);
>> }
>>
>> /*
>
> Thanks,
> Kirill
