linux-kernel - Re: [PATCH] mm/filemap: invalidating pages is still necessary when io with IOCB

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Date: Thu, 23 May 2024 22:11:02 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	Goldwyn Rodrigues <rgoldwyn@...e.com>,
	Christoph Hellwig <hch@....de>, Jan Kara <jack@...e.cz>,
	Jens Axboe <axboe@...nel.dk>, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] mm/filemap: invalidating pages is still necessary when
 io with IOCB_NOWAIT

On Thu, May 23, 2024 at 01:08:02PM -0700, Andrew Morton wrote:
> On Mon, 13 May 2024 21:23:39 +0800 Liu Wei <liuwei09@...tc.cn> wrote:
> 
> > After commit (6be96d3ad3 fs: return if direct I/O will trigger writeback),
> > when we issuing AIO with direct I/O and IOCB_NOWAIT on a block device, the
> > process context will not be blocked.
> > 
> > However, if the device already has page cache in memory, EAGAIN will be
> > returned. And even when trying to reissue the AIO with direct I/O and
> > IOCB_NOWAIT again, we consistently receive EAGAIN.
> > 
> > Maybe a better way to deal with it: filemap_fdatawrite_range dirty pages
> > with WB_SYNC_NONE flag, and invalidate_mapping_pages unmapped pages at
> > the same time.
> 
> Can't userspace do this?  If EAGAIN, sync the fd and retry the IO?

I don't think that it can, because the pages will still be there, even
if now clean?  I think the idea was to punt to a worker thread which
could sleep and retry without NOWAIT.  But let's see what someone
involved in this patch has to say about the intent.