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

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Date:   Wed, 16 Nov 2022 13:24:40 +0100
From:   David Sterba <dsterba@...e.cz>
To:     Qu Wenruo <quwenruo.btrfs@....com>
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 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.