On 2023/4/26 19:07, 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);
}
Is it tested with a pure ext4 without any other fs background?
I don't think it's true that "ext4->readahead() doesn't call
alloc_page()" since I think even ext2/ext4 uses buffer head
interfaces to read metadata (extents or old block mapping)
from its bd_inode for readahead, which indirectly allocates
some extra pages to page cache as well.
The difference only here is the total number of pages to be
allocated here, but many extra compressed data takeing extra
allocation causes worse. So I think it much depends on how
stressful does your stress workload work like, and I'm even
not sure it's a real issue since if you stop the stress
workload, it will immediately recover (only it may not oom
directly).
Thanks,
Gao Xiang
