Re: [PATCH] erofs: fix unexpected EIO under memory pressure

[Gao Xiang <hsiangkao@linux.alibaba.com>]

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: 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>
> ---
>   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.

Thanks,
Gao Xiang

>   	}
>   	return err;
>   }