linux-kernel - Re: [PATCH] percpu: make pcpu_alloc() aware of current gfp context

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <20200430144018.c855f031b321d68e5c89b30c@linux-foundation.org>
Date:   Thu, 30 Apr 2020 14:40:18 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     fdmanana@...nel.org
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        dennis@...nel.org, tj@...nel.org, cl@...ux.com,
        linux-btrfs@...r.kernel.org, Filipe Manana <fdmanana@...e.com>
Subject: Re: [PATCH] percpu: make pcpu_alloc() aware of current gfp context

On Thu, 30 Apr 2020 17:43:56 +0100 fdmanana@...nel.org wrote:

> From: Filipe Manana <fdmanana@...e.com>
> 
> Since 5.7-rc1, on btrfs we have a percpu counter initialization for which
> we always pass a GFP_KERNEL gfp_t argument (this happens since commit
> 2992df73268f78 ("btrfs: Implement DREW lock")).  That is safe in some
> contextes but not on others where allowing fs reclaim could lead to a
> deadlock because we are either holding some btrfs lock needed for a
> transaction commit or holding a btrfs transaction handle open.  Because
> of that we surround the call to the function that initializes the percpu
> counter with a NOFS context using memalloc_nofs_save() (this is done at
> btrfs_init_fs_root()).
> 
> However it turns out that this is not enough to prevent a possible
> deadlock because percpu_alloc() determines if it is in an atomic context
> by looking exclusively at the gfp flags passed to it (GFP_KERNEL in this
> case) and it is not aware that a NOFS context is set.  Because it thinks
> it is in a non atomic context it locks the pcpu_alloc_mutex, which can
> result in a btrfs deadlock when pcpu_balance_workfn() is running, has
> acquired that mutex and is waiting for reclaim, while the btrfs task that
> called percpu_counter_init() (and therefore percpu_alloc()) is holding
> either the btrfs commit_root semaphore or a transaction handle (done at
> fs/btrfs/backref.c:iterate_extent_inodes()), which prevents reclaim from
> finishing as an attempt to commit the current btrfs transaction will
> deadlock.
> 

Patch looks good and seems sensible, thanks.

But why did btrfs use memalloc_nofs_save()/restore() rather than
s/GFP_KERNEL/GFP_NOFS/?