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

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Message-ID: <3477941.iIbC2pHGDl@suse>
Date:   Thu, 12 Jan 2023 21:42:33 +0100
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Jeff Moyer <jmoyer@...hat.com>, Al Viro <viro@...iv.linux.org.uk>
Cc:     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 mercoledì 11 gennaio 2023 17:02:26 CET Al Viro wrote:
> 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...

Actually, I'd prefer Ira's solution for readability, but I have nothing 
against yours. I will send you the code in the way you rewrote it.

> > >>  	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.

Yes, I missed it :-(

However, with the use of memcpy_to_page() we no longer need that explicit call 
to flush_dcache_page().

Thank you,

Fabio