linux-ext4 - Re: [PATCH] ext4: Prevent ext4_kvmalloc re-entring the filesystem and deadlocking

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Date:   Wed, 8 Jan 2020 17:11:13 +0900
From:   Naoto Kobayashi <naoto.kobayashi4c@...il.com>
To:     "Theodore Y. Ts'o" <tytso@....edu>
Cc:     adilger.kernel@...ger.ca, linux-ext4@...r.kernel.org
Subject: Re: [PATCH] ext4: Prevent ext4_kvmalloc re-entring the filesystem and
 deadlocking

On 2019/12/26 23:39, Theodore Y. Ts'o wrote:
> On Thu, Dec 26, 2019 at 04:10:08PM +0900, Naoto Kobayashi wrote:
>> Although __vmalloc() doesn't support GFP_NOFS[1],
>> ext4_kvmalloc/kvzalloc caller may be under GFP_NOFS context
>> (e.g. fs/ext4/resize.c::add_new_gdb). In such cases, the memory
>> reclaim can re-entr the filesystem and potentially deadlock.
>>
>> To prevent the memory relcaim re-entring the filesystem,
>> use memalloc_nofs_save/restore that gets __vmalloc() to drop
>> __GFP_FS flag.
>>
>> [1] linux-tree/Documentation/core-api/gfp-mask-fs-io.rst
>>
>> Signed-off-by: Naoto Kobayashi <naoto.kobayashi4c@...il.com>
> 
> Good catch!  However, we're not actually using ext4_kvzalloc()
> anywhere, at least not any more.  And also, all the users of
> ext4_kvmalloc() are using GFP_NOFS (otherwise, they would have been
> converted over to use the generic kvmalloc() helper function).
> 
> So... a cleaner fix would be to (a) delete ext4_kvmazalloc(), (b)
> rename ext4_kvmalloc() to ext4_kvmalloc_nofs(), and drop its flags
> argument, and then the calls memalloc_nfs_save/restore() to
> ext4_kvmalloc_nofs() --- and so we won't need the
> 
> 	if (!(flags & __GFP_FS))
> 
> test.
> 
> Could you make those changes and resend the patch?
> 
> Thanks,
> 
> 					- Ted
> 

Thank you for the review! I made the changes you suggested and resent
the patch.

  [PATCH v2 0/3] ext4: Prevent memory reclaim from re-entering the filesystem and deadlocking
  https://marc.info/?l=linux-ext4&m=157743393000807&w=2

Would you review the patch?

Thanks,

                                       - Naoto