linux-kernel - Re: [PATCH v3] fs/buffer.c: update per-CPU bh

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Date:   Fri, 3 Feb 2023 09:36:53 +1100
From:   Dave Chinner <david@...morbit.com>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Matthew Wilcox <willy@...radead.org>,
        Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
        Frederic Weisbecker <frederic@...nel.org>,
        Leonardo Bras <leobras@...hat.com>,
        Yair Podemsky <ypodemsk@...hat.com>, P J P <ppandit@...hat.com>
Subject: Re: [PATCH v3] fs/buffer.c: update per-CPU bh_lru cache via RCU

On Wed, Feb 01, 2023 at 01:01:47PM -0300, Marcelo Tosatti wrote:
> 
> umount calls invalidate_bh_lrus which IPIs each

via invalidate_bdev(). So this is only triggered on unmount of
filesystems that use the block device mapping directly, right?

Or is the problem that userspace is polling the block device (e.g.
udisks, blkid, etc) whilst the filesystem is mounted and populating
the block device mapping with cached pages so invalidate_bdev()
always does work even when the filesystem doesn't actually use the
bdev mapping?

> CPU that has non empty per-CPU buffer_head cache:
> 
>        	on_each_cpu_cond(has_bh_in_lru, invalidate_bh_lru, NULL, 1);
> 
> This interrupts CPUs which might be executing code sensitive
> to interferences.
> 
> To avoid the IPI, free the per-CPU caches remotely via RCU.
> Two bh_lrus structures for each CPU are allocated: one is being
> used (assigned to per-CPU bh_lru pointer), and the other is
> being freed (or idle).

Rather than adding more complexity to the legacy bufferhead code,
wouldn't it be better to switch the block device mapping to use
iomap+folios and get rid of the use of bufferheads altogether?

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com