linux-kernel - Re: [PATCH v2] fs/aio: Replace kmap{,_atomic}() with kmap_local

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Date:   Wed, 11 Jan 2023 16:02:26 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Jeff Moyer <jmoyer@...hat.com>
Cc:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        Benjamin LaHaise <bcrl@...ck.org>, linux-aio@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Venkataramanan, Anirudh" <anirudh.venkataramanan@...el.com>,
        Ira Weiny <ira.weiny@...el.com>
Subject: Re: [PATCH v2] fs/aio: Replace kmap{,_atomic}() with
 kmap_local_page()

On Wed, Jan 11, 2023 at 09:13:40AM -0500, Jeff Moyer wrote:
> Hi, Al,
> 
> Al Viro <viro@...iv.linux.org.uk> writes:
> 
> > On Mon, Jan 09, 2023 at 06:56:29PM +0100, Fabio M. De Francesco wrote:
> >
> >> -	ring = kmap_atomic(ctx->ring_pages[0]);
> >> +	ring = kmap_local_page(ctx->ring_pages[0]);
> >>  	ring->nr = nr_events;	/* user copy */
> >>  	ring->id = ~0U;
> >>  	ring->head = ring->tail = 0;
> >> @@ -575,7 +575,7 @@ static int aio_setup_ring(struct kioctx *ctx, unsigned int nr_events)
> >>  	ring->compat_features = AIO_RING_COMPAT_FEATURES;
> >>  	ring->incompat_features = AIO_RING_INCOMPAT_FEATURES;
> >>  	ring->header_length = sizeof(struct aio_ring);
> >> -	kunmap_atomic(ring);
> >> +	kunmap_local(ring);
> >>  	flush_dcache_page(ctx->ring_pages[0]);
> >
> > I wonder if it would be more readable as memcpy_to_page(), actually...
> 
> I'm not sure I understand what you're suggesting.

	memcpy_to_page(ctx->ring_pages[0], 0, &(struct aio_ring){
			.nr = nr_events, .id = ~0U, .magic = AIO_RING_MAGIC,
			.compat_features = AIO_RING_COMPAT_FEATURES,
			.in_compat_features = AIO_RING_INCOMPAT_FEATURES,
			.header_length = sizeof(struct aio_ring)},
			sizeof(struct aio_ring));

instead of the lines from kmap_atomic to flush_dcache_page...

> 
> >>  
> >>  	return 0;
> >> @@ -678,9 +678,9 @@ static int ioctx_add_table(struct kioctx *ctx, struct mm_struct *mm)
> >>  					 * we are protected from page migration
> >>  					 * changes ring_pages by ->ring_lock.
> >>  					 */
> >> -					ring = kmap_atomic(ctx->ring_pages[0]);
> >> +					ring = kmap_local_page(ctx->ring_pages[0]);
> >>  					ring->id = ctx->id;
> >> -					kunmap_atomic(ring);
> >> +					kunmap_local(ring);
> >
> > Incidentally, does it need flush_dcache_page()?
> 
> Yes, good catch.