linux-kernel - Re: [PATCH] fs/iomap: Fix write path page prefaulting

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Date:   Tue, 23 Nov 2021 17:29:18 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Andreas Gruenbacher <agruenba@...hat.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Christoph Hellwig <hch@...radead.org>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs/iomap: Fix write path page prefaulting

On Tue, Nov 23, 2021 at 04:18:12PM +0100, Andreas Gruenbacher wrote:
> When part of the user buffer passed to generic_perform_write() or
> iomap_file_buffered_write() cannot be faulted in for reading, the entire
> write currently fails.
> 
> The correct behavior would be to write all the data that can be written,
> up to the point of failure.  Since commit a6294593e8a1 ("iov_iter: Turn
> iov_iter_fault_in_readable into fault_in_iov_iter_readable"), we have
> enough information to implement that, so change the code to do that.
> 
> We already take into account that pages faulted in may no longer be
> resident by the time they are accessed, so the code will also behave
> correctly when part of the buffer isn't faulted in in the first place.
> 
> This leads to an intentional user-visible change when the buffer passed
> to write calls contains unmapped or poisoned pages.
> 
> (This change obsoletes commit 554c577cee95 ("gfs2: Prevent endless loops
> in gfs2_file_buffered_write").)
> 
> Signed-off-by: Andreas Gruenbacher <agruenba@...hat.com>

FWIW:

Reviewed-by: Catalin Marinas <catalin.marinas@....com>

This would be more consistent as in my tests on a vanilla kernel ext4
and btrfs fail with EFAULT while gfs2 copies as much as it can before
hitting the fault.

-- 
Catalin