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

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Date:   Thu, 22 Nov 2018 14:40:50 +0900
From:   Eiichi Tsukata <devel@...ukata.com>
To:     Alexander Viro <viro@...iv.linux.org.uk>
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

2018年11月21日(水) 13:54 Al Viro <viro@...iv.linux.org.uk>:
>
> 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).

Thanks.

The lock I added in NFS was nothing but slow down lseek() because a file size is
updated atomically. Even `spin_lock(&inode->i_lock)` is unnecessary.

I'll fix the commit message which only refers to specific local file
systems that use
generic_perform_write() and remove unnecessary locks in some
distributed file systems
(e.g. nfs, cifs, or more) by replacing generic_file_llseek() with
generic_file_llseek_unlocked()
so that `tail` don't have to wait for avian carriers.