On Tue, Dec 30, 2025 at 10:30:53AM +0800, Gao Xiang wrote: > From: Junbeom Yeom <[email protected]> > > 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! > ... > > 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]> > Reviewed-by: Gao Xiang <[email protected]> > Signed-off-by: Gao Xiang <[email protected]> > --- > Hi Greg and Sasha, > > Let's just merge this directly. > No need to backport commit 831faabed812 ("erofs: improve decompression error > reporting") > for now.
Now taken, thanks! greg k-h
