Hi Junbeom,
On 2025/12/19 15:10, Junbeom Yeom wrote:
erofs readahead could fail with ENOMEM under the memory pressure
because it tries to alloc_page with GFP_NOWAIT | GFP_NORETRY, while
GFP_KERNEL for a regular read. And if readahead fails (with
non-uptodate folios), the original request will then fall back to
synchronous read, and `.read_folio()` should return appropriate errnos.
However, in scenarios where readahead and read operations compete,
read operation could return an unintended EIO because of an incorrect
error propagation.
To resolve this, this patch modifies the behavior so that, when the
PCL is for read(which means pcl.besteffort is true), it attempts
actual decompression instead of propagating the privios error except initial
EIO.
- Page size: 4K
- The original size of FileA: 16K
- Compress-ratio per PCL: 50% (Uncompressed 8K -> Compressed 4K)
[page0, page1] [page2, page3] [PCL0]---------[PCL1]
- functions declaration:
. pread(fd, buf, count, offset)
. readahead(fd, offset, count)
- Thread A tries to read the last 4K
- Thread B tries to do readahead 8K from 4K
- RA, besteffort == false
- R, besteffort == true
<process A> <process B>
pread(FileA, buf, 4K, 12K)
do readahead(page3) // failed with ENOMEM
wait_lock(page3)
if (!uptodate(page3))
goto do_read
readahead(FileA, 4K, 8K)
// Here create PCL-chain like below:
// [null, page1] [page2, null]
// [PCL0:RA]-----[PCL1:RA]
...
do read(page3) // found [PCL1:RA] and add page3 into it,
// and then, change PCL1 from RA to R ...
// Now, PCL-chain is as below:
// [null, page1] [page2, page3]
// [PCL0:RA]-----[PCL1:R]
// try to decompress PCL-chain...
z_erofs_decompress_queue
err = 0;
// failed with ENOMEM, so page 1
// only for RA will not be uptodated.
// it's okay.
err = decompress([PCL0:RA], err)
// However, ENOMEM propagated to next
// PCL, even though PCL is not only
// for RA but also for R. As a result,
// it just failed with ENOMEM without
// trying any decompression, so page2
// and page3 will not be uptodated.
** BUG HERE ** --> err = decompress([PCL1:R], err)
return err as ENOMEM ...
wait_lock(page3)
if (!uptodate(page3))
return EIO <-- Return an unexpected EIO!
...
Many thanks for the report!
It's indeed a new issue to me.
Fixes: 2349d2fa02db ("erofs: sunset unneeded NOFAILs")
Cc: [email protected]
Reviewed-by: Jaewook Kim <[email protected]>
Reviewed-by: Sungjong Seo <[email protected]>
Signed-off-by: Junbeom Yeom <[email protected]>
---
fs/erofs/zdata.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)
diff --git a/fs/erofs/zdata.c b/fs/erofs/zdata.c index
27b1f44d10ce..86bf6e087d34 100644
--- a/fs/erofs/zdata.c
+++ b/fs/erofs/zdata.c
@@ -1414,11 +1414,15 @@ static int z_erofs_decompress_queue(const struct
z_erofs_decompressqueue *io,
};
struct z_erofs_pcluster *next;
int err = io->eio ? -EIO : 0;
+ int io_err = err;
for (; be.pcl != Z_EROFS_PCLUSTER_TAIL; be.pcl = next) {
+ int propagate_err;
+
DBG_BUGON(!be.pcl);
next = READ_ONCE(be.pcl->next);
- err = z_erofs_decompress_pcluster(&be, err) ?: err;
+ propagate_err = READ_ONCE(be.pcl->besteffort) ? io_err : err;
+ err = z_erofs_decompress_pcluster(&be, propagate_err) ?: err;
I wonder if it's just possible to decompress each pcluster according to io
status only (but don't bother with previous pcluster status), like:
err = z_erofs_decompress_pcluster(&be, io->eio) ?: err;
and change the second argument of
z_erofs_decompress_pcluster() to bool.
So that we could leverage the successful i/o as much as possible.
