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SubjectRe: [PATCH v4 1/3] btrfs: add might_sleep() to some places in update_qgroup_limit_item()
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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@suse.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

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