linux-ext4 - Re: [PATCH v1 0/4] fs: fix race between llseek SEEK

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Date:   Wed, 21 Nov 2018 04:54:41 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Eiichi Tsukata <devel@...ukata.com>
Cc:     andi@...stfloor.org, Chris Mason <clm@...com>,
        Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>, Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Jaegeuk Kim <jaegeuk@...nel.org>, Chao Yu <yuchao0@...wei.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        Bob Peterson <rpeterso@...hat.com>,
        Andreas Gruenbacher <agruenba@...hat.com>,
        linux-btrfs@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-fsdevel@...r.kernel.org, cluster-devel@...hat.com,
        linux-unionfs@...r.kernel.org
Subject: Re: [PATCH v1 0/4] fs: fix race between llseek SEEK_END and write

On Wed, Nov 21, 2018 at 11:43:56AM +0900, Eiichi Tsukata wrote:
> Some file systems (including ext4, xfs, ramfs ...) have the following
> problem as I've described in the commit message of the 1/4 patch.
> 
>   The commit ef3d0fd27e90 ("vfs: do (nearly) lockless generic_file_llseek")
>   removed almost all locks in llseek() including SEEK_END. It based on the
>   idea that write() updates size atomically. But in fact, write() can be
>   divided into two or more parts in generic_perform_write() when pos
>   straddles over the PAGE_SIZE, which results in updating size multiple
>   times in one write(). It means that llseek() can see the size being
>   updated during write().

And?  Who has ever promised anything that insane?  write(2) can take an arbitrary
amount of time; another process doing lseek() on independently opened descriptor
is *not* going to wait for that (e.g. page-in of the buffer being written, which
just happens to be mmapped from a file on NFS over RFC1149 link).