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).
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
