linux-ext4 - Re: ext2, possible circular locking dependency detected

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Date:   Mon, 24 Feb 2020 19:02:16 +0900
From:   "J. R. Okajima" <hooanon05g@...il.com>
To:     Jan Kara <jack@...e.cz>
Cc:     jack@...e.com, linux-ext4@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: ext2, possible circular locking dependency detected

Jan Kara:
> This is not the right way how memalloc_nofs_save() should be used (you
> could just use GFP_NOFS instead of GFP_KERNEL instead of wrapping the
> allocation inside memalloc_nofs_save/restore()). The
> memalloc_nofs_save/restore() API is created so that you can change the
> allocation context at the place which mandates the new context - i.e., in
> this case when acquiring / dropping xattr_sem. That way you don't have to
> propagate the context information down to function calls and the code is
> also future-proof - if you add new allocation, they will use correct
> allocation context.

Thanks for the lecture about memalloc_nofs_save/restore().
Honestly speaking, I didn't know these APIs and I always use GFP_NOFS
flag. Investigating this lockdep warning, I read the comments in gfp.h.

 * %GFP_NOFS will use direct reclaim but will not use any filesystem interfaces.
 * Please try to avoid using this flag directly and instead use
 * memalloc_nofs_{save,restore} to mark the whole scope which cannot/shouldn't
 * recurse into the FS layer with a short explanation why. All allocation
 * requests will inherit GFP_NOFS implicitly.

Actually grep-ping the whole kernel source tree told me there are
several "one-liners" like ...nofs_save(); kmalloc(); ...nofs_restore
sequence.  But re-reading the comments and your mail, I understand these
APIs are for much wider region than such one-liner.

I don't think it a good idea that I send you another patch replaced by
GFP_NOFS.  You can fix it simply and you know much more than me about
this matter, and I will be satisfied when this problem is fixed by you.

J. R. Okajima