Re: [PATCH 6.18.y v2] erofs: fix unexpected EIO under memory pressure

[Greg KH <gregkh@linuxfoundation.org>]

On Tue, Dec 30, 2025 at 10:30:53AM +0800, Gao Xiang wrote:
> From: Junbeom Yeom <junbeom.yeom@samsung.com>
> 
> 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: stable@vger.kernel.org
> Reviewed-by: Jaewook Kim <jw5454.kim@samsung.com>
> Reviewed-by: Sungjong Seo <sj1557.seo@samsung.com>
> Signed-off-by: Junbeom Yeom <junbeom.yeom@samsung.com>
> Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
> Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
> ---
> 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