Re: [PATCH] gfs2: use __vmalloc GFP_NOFS for fs-related allocations.

Steven Whitehouse
2015-02-02T10:30:42Z

Hi,

On 02/02/15 08:11, Dave Chinner wrote:
> On Mon, Feb 02, 2015 at 01:57:23AM -0500, Oleg Drokin wrote:
>> Hello!
>>
>> On Feb 2, 2015, at 12:37 AM, Dave Chinner wrote:
>>
>>> On Sun, Feb 01, 2015 at 10:59:54PM -0500, green@linuxhacker.ru wrote:
>>>> From: Oleg Drokin <green@linuxhacker.ru>
>>>>
>>>> leaf_dealloc uses vzalloc as a fallback to kzalloc(GFP_NOFS), so
>>>> it clearly does not want any shrinker activity within the fs itself.
>>>> convert vzalloc into __vmalloc(GFP_NOFS|__GFP_ZERO) to better achieve
>>>> this goal.
>>>>
>>>> Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
>>>> ---
>>>> fs/gfs2/dir.c | 3 ++-
>>>> 1 file changed, 2 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/fs/gfs2/dir.c b/fs/gfs2/dir.c
>>>> index c5a34f0..6371192 100644
>>>> --- a/fs/gfs2/dir.c
>>>> +++ b/fs/gfs2/dir.c
>>>> @@ -1896,7 +1896,8 @@ static int leaf_dealloc(struct gfs2_inode *dip, u32 index, u32 len,
>>>>
>>>> ht = kzalloc(size, GFP_NOFS | __GFP_NOWARN);
>>>> if (ht == NULL)
>>>> - ht = vzalloc(size);
>>>> + ht = __vmalloc(size, GFP_NOFS | __GFP_NOWARN | __GFP_ZERO,
>>>> + PAGE_KERNEL);
>>> That, in the end, won't help as vmalloc still uses GFP_KERNEL
>>> allocations deep down in the PTE allocation code. See the hacks in
>>> the DM and XFS code to work around this. i.e. go look for callers of
>>> memalloc_noio_save(). It's ugly and grotesque, but we've got no
>>> other way to limit reclaim context because the MM devs won't pass
>>> the vmalloc gfp context down the stack to the PTE allocations....
>> Hm, interesting.
>> So all the other code in the kernel that does this sort of thing (and there's quite a bit
>> outside of xfs and ocfs2) would not get the desired effect?
> No. I expect, however, that very few people would ever see a
> deadlock as a result - it's a pretty rare sort of kernel case to hit
> in most cases. XFS does make extensive use of vm_map_ram() in
> GFP_NOFS context, however, when large directory block sizes are in
> use, and we also have a history of lockdep throwing warnings under
> memory pressure. In the end, the memalloc_noio_save() changes were
> made to stop the frequent lockdep reports rather than actual
> deadlocks.
Indeed, I think the patch is still an improvement however, so I'm happy
to apply it while a better solution is found.

>> So, I did some digging in archives and found this thread from 2010 onward with various
>> patches and rants.
>> Not sure how I missed that before.
>>
>> Should we have another run at this I wonder?
> By all means, but I don't think you'll have any more luck than
> anyone else in the past. We've still got the problem of attitude
> ("vmalloc is not for general use") and making it actually work is
> seen as "encouraging undesirable behaviour". If you can change
> attitudes towards vmalloc first, then you'll be much more likely to
> make progress in getting these problems solved....
>

Well I don't know whether it has to be vmalloc that provides the
solution here... if memory fragmentation could be controlled then
kmalloc of larger contiguous chunks of memory could be done using that,
which might be a better solution overall. But I do agree that we need to
try and come to some kind of solution to this problem as it is one of
those things that has been rumbling on for a long time without a proper
solution.

I also wonder if vmalloc is still very slow? That was the case some time
ago when I noticed a problem in directory access times in gfs2, which
made us change to use kmalloc with a vmalloc fallback in the first place,

Steve.