linux-kernel - Re: [PATCH v4 1/3] btrfs: add might_sleep() to some places in update_qgroup_limit

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Message-ID: <7b2ac1a3-4556-1a37-8442-6b7a51b2dc84@gmx.com>
Date:   Wed, 16 Nov 2022 20:26:46 +0800
From:   Qu Wenruo <quwenruo.btrfs@....com>
To:     dsterba@...e.cz
Cc:     ChenXiaoSong <chenxiaosong2@...wei.com>, clm@...com,
        josef@...icpanda.com, dsterba@...e.com,
        linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org,
        zhangxiaoxu5@...wei.com, yanaijie@...wei.com, wqu@...e.com
Subject: Re: [PATCH v4 1/3] btrfs: add might_sleep() to some places in
 update_qgroup_limit_item()



On 2022/11/16 20:24, David Sterba wrote:
> On Wed, Nov 16, 2022 at 04:43:50PM +0800, Qu Wenruo wrote:
>>
>>
>> On 2022/11/16 16:09, ChenXiaoSong wrote:
>>> 在 2022/11/16 6:48, Qu Wenruo 写道:
>>>> Looks good.
>>>>
>>>> We may want to add more in other locations, but this is really a good
>>>> start.
>>>>
>>>> Reviewed-by: Qu Wenruo <wqu@...e.com>
>>>>
>>>> Thanks,
>>>> Qu
>>>
>>> If I just add might_sleep() in btrfs_alloc_path() and
>>> btrfs_search_slot(), is it reasonable?
>>
>> Adding it to btrfs_search_slot() is definitely correct.
>>
>> But why for btrfs_alloc_path()? Wouldn't kmem_cache_zalloc() itself
>> already do the might_sleep_if() somewhere?
>>
>> I just looked the call chain, and indeed it is doing the check already:
>>
>> btrfs_alloc_path()
>> |- kmem_cache_zalloc()
>>      |- kmem_cache_alloc()
>>         |- __kmem_cache_alloc_lru()
>>            |- slab_alloc()
>>               |- slab_alloc_node()
>>                  |- slab_pre_alloc_hook()
>>                     |- might_alloc()
>>                        |- might_sleep_if()
> 
> The call chaing is unconditional so the check will always happen but the
> condition itself in might_sleep_if does not recognize GFP_NOFS:
> 
>   34 static inline bool gfpflags_allow_blocking(const gfp_t gfp_flags)
>   35 {
>   36         return !!(gfp_flags & __GFP_DIRECT_RECLAIM);
>   37 }
> 
> #define GFP_NOFS        (__GFP_RECLAIM | __GFP_IO)
> 
> And I think the qgroup limit was exactly a spin lock over btrfs_path_alloc so
> it did not help. An might_sleep() inside btrfs_path_alloc() is a very minimal
> but reliable check we could add, the paths are used in many places so it would
> increase the coverage.

OK, then it makes sense now for btrfs_alloc_path().

But I still believe this looks like a bug in gfpflags_allow_blocking()...

Thanks,
Qu