On 4/27/23 03:34, Phillip Lougher wrote:
On 26/04/2023 20:06, Phillip Lougher wrote:
On 26/04/2023 19:26, Yang Shi wrote:
On Wed, Apr 26, 2023 at 10:38 AM Phillip Lougher
<phillip@xxxxxxxxxxxxxxx> wrote:
On 26/04/2023 17:44, Phillip Lougher wrote:
On 26/04/2023 12:07, Hui Wang wrote:
On 4/26/23 16:33, Michal Hocko wrote:
[CC squashfs maintainer]
On Wed 26-04-23 13:10:30, Hui Wang wrote:
If we run the stress-ng in the filesystem of squashfs, the system
will be in a state something like hang, the stress-ng couldn't
finish running and the console couldn't react to users' input.
This issue happens on all arm/arm64 platforms we are working on,
through debugging, we found this issue is introduced by oom
handling
in the kernel.
The fs->readahead() is called between memalloc_nofs_save() and
memalloc_nofs_restore(), and the squashfs_readahead() calls
alloc_page(), in this case, if there is no memory left, the
out_of_memory() will be called without __GFP_FS, then the oom
killer
will not be triggered and this process will loop endlessly and
wait
for others to trigger oom killer to release some memory. But for a
system with the whole root filesystem constructed by squashfs,
nearly all userspace processes will call out_of_memory() without
__GFP_FS, so we will see that the system enters a state
something like
hang when running stress-ng.
To fix it, we could trigger a kthread to call page_alloc() with
__GFP_FS before returning from out_of_memory() due to without
__GFP_FS.
I do not think this is an appropriate way to deal with this issue.
Does it even make sense to trigger OOM killer for something like
readahead? Would it be more mindful to fail the allocation instead?
That being said should allocations from squashfs_readahead use
__GFP_RETRY_MAYFAIL instead?
Thanks for your comment, and this issue could hardly be
reproduced on
ext4 filesystem, that is because the ext4->readahead() doesn't call
alloc_page(). If changing the ext4->readahead() as below, it will be
easy to reproduce this issue with the ext4 filesystem (repeatedly
run: $stress-ng --bigheap ${num_of_cpu_threads} --sequential 0
--timeout 30s --skip-silent --verbose)
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index ffbbd9626bd8..8b9db0b9d0b8 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -3114,12 +3114,18 @@ static int ext4_read_folio(struct file
*file,
struct folio *folio)
static void ext4_readahead(struct readahead_control *rac)
{
struct inode *inode = rac->mapping->host;
+ struct page *tmp_page;
/* If the file has inline data, no need to do readahead. */
if (ext4_has_inline_data(inode))
return;
+ tmp_page = alloc_page(GFP_KERNEL);
+
ext4_mpage_readpages(inode, rac, NULL);
+
+ if (tmp_page)
+ __free_page(tmp_page);
}
BTW, I applied my patch to the linux-next and ran the oom stress-ng
testcases overnight, there is no hang, oops or crash, looks like
there is no big problem to use a kthread to trigger the oom
killer in
this case.
And Hi squashfs maintainer, I checked the code of filesystem, looks
like most filesystems will not call alloc_page() in the readahead(),
could you please help take a look at this issue, thanks.
This will be because most filesystems don't need to do so.
Squashfs is
a compressed filesystem with large blocks covering much more than one
page, and it decompresses these blocks in squashfs_readahead(). If
__readahead_batch() does not return the full set of pages covering
the
Squashfs block, it allocates a temporary page for the
decompressors to
decompress into to "fill in the hole".
What can be done here as far as Squashfs is concerned .... I could
move the page allocation out of the readahead path (e.g. do it at
mount time).
You could try this patch which does that. Compile tested only.
The kmalloc_array() may call alloc_page() to trigger this problem too
IIUC. It should be pre-allocated as well?
That is a much smaller allocation, and so it entirely depends whether
it is an issue or not. There are also a number of other small memory
allocations in the path as well.
The whole point of this patch is to move the *biggest* allocation
which is the reported issue and then see what happens. No point in
making this test patch more involved and complex than necessary at
this point.
Phillip
Also be aware this stress-ng triggered issue is new, and apparently
didn't occur last year. So it is reasonable to assume the issue has
been introduced as a side effect of the readahead improvements. One
of these is this allocation of a temporary page to decompress into
rather than falling back to entirely decompressing into a
pre-allocated buffer (allocated at mount time). The small memory
allocations have been there for many years.
Allocating the page at mount time effectively puts the memory
allocation situation back to how it was last year before the readahead
work.
Phillip
Thanks Phillip and Yang.
And Phillip,
I tested your change, it didn't help. According to my debug, the OOM
happens at the place of allocating memory for bio, it is at the line of
"struct page *page = alloc_page(GFP_NOIO);" in the squashfs_bio_read().
Other filesystems just use the pre-allocated memory in the "struct
readahead_control" to do the bio, but squashfs allocate the new page to
do the bio (maybe because the squashfs is a compressed filesystem).
BTW, this is not a new issue for squashfs, we have uc20 (linux-5.4
kernel) and uc22 (linux-5.15 kernel), all have this issue. The issue
already existed in the squahsfs_readpage() in the 5.4 kernel.
I guess if could use pre-allocated memory to do the bio, it will help.
Thanks,
Hui.
fs/squashfs/page_actor.c | 10 +---------
fs/squashfs/page_actor.h | 1 -
fs/squashfs/squashfs_fs_sb.h | 1 +
fs/squashfs/super.c | 10 ++++++++++
4 files changed, 12 insertions(+), 10 deletions(-)
diff --git a/fs/squashfs/page_actor.c b/fs/squashfs/page_actor.c
index 81af6c4ca115..6cce239eca66 100644
--- a/fs/squashfs/page_actor.c
+++ b/fs/squashfs/page_actor.c
@@ -110,15 +110,7 @@ struct squashfs_page_actor
*squashfs_page_actor_init_special(struct squashfs_sb_
if (actor == NULL)
return NULL;
- if (msblk->decompressor->alloc_buffer) {
- actor->tmp_buffer = kmalloc(PAGE_SIZE, GFP_KERNEL);
-
- if (actor->tmp_buffer == NULL) {
- kfree(actor);
- return NULL;
- }
- } else
- actor->tmp_buffer = NULL;
+ actor->tmp_buffer = msblk->actor_page;
actor->length = length ? : pages * PAGE_SIZE;
actor->page = page;
diff --git a/fs/squashfs/page_actor.h b/fs/squashfs/page_actor.h
index 97d4983559b1..df5e999afa42 100644
--- a/fs/squashfs/page_actor.h
+++ b/fs/squashfs/page_actor.h
@@ -34,7 +34,6 @@ static inline struct page
*squashfs_page_actor_free(struct squashfs_page_actor *
{
struct page *last_page = actor->last_page;
- kfree(actor->tmp_buffer);
kfree(actor);
return last_page;
}
diff --git a/fs/squashfs/squashfs_fs_sb.h
b/fs/squashfs/squashfs_fs_sb.h
index 72f6f4b37863..8feddc9e6cce 100644
--- a/fs/squashfs/squashfs_fs_sb.h
+++ b/fs/squashfs/squashfs_fs_sb.h
@@ -47,6 +47,7 @@ struct squashfs_sb_info {
struct squashfs_cache *block_cache;
struct squashfs_cache *fragment_cache;
struct squashfs_cache *read_page;
+ void *actor_page;
int next_meta_index;
__le64 *id_table;
__le64 *fragment_index;
diff --git a/fs/squashfs/super.c b/fs/squashfs/super.c
index e090fae48e68..674dc187d961 100644
--- a/fs/squashfs/super.c
+++ b/fs/squashfs/super.c
@@ -329,6 +329,15 @@ static int squashfs_fill_super(struct
super_block *sb, struct fs_context *fc)
goto failed_mount;
}
+
+ /* Allocate page for
squashfs_readahead()/squashfs_read_folio() */
+ if (msblk->decompressor->alloc_buffer) {
+ msblk->actor_page = kmalloc(PAGE_SIZE, GFP_KERNEL);
+
+ if(msblk->actor_page == NULL)
+ goto failed_mount;
+ }
+
msblk->stream = squashfs_decompressor_setup(sb, flags);
if (IS_ERR(msblk->stream)) {
err = PTR_ERR(msblk->stream);
@@ -454,6 +463,7 @@ static int squashfs_fill_super(struct
super_block *sb, struct fs_context *fc)
squashfs_cache_delete(msblk->block_cache);
squashfs_cache_delete(msblk->fragment_cache);
squashfs_cache_delete(msblk->read_page);
+ kfree(msblk->actor_page);
msblk->thread_ops->destroy(msblk);
kfree(msblk->inode_lookup_table);
kfree(msblk->fragment_index);
--
2.35.1
Adding __GFP_RETRY_MAYFAIL so the alloc() can fail will mean Squashfs
returning I/O failures due to no memory. That will cause a lot of
applications to crash in a low memory situation. So a crash rather
than a hang.
Phillip
