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From:   "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
To:     Alexander Viro <viro@zeniv.linux.org.uk>,
        Benjamin LaHaise <bcrl@kvack.org>,
        linux-fsdevel@vger.kernel.org, linux-aio@kvack.org,
        linux-kernel@vger.kernel.org
Cc:     "Venkataramanan, Anirudh" <anirudh.venkataramanan@intel.com>,
        Ira Weiny <ira.weiny@intel.com>, Jeff Moyer <jmoyer@redhat.com>
Subject: Re: [RESEND PATCH] fs/aio: Replace kmap{,_atomic}() with kmap_local_page()
Date:   Sat, 26 Nov 2022 17:51:47 +0100
Message-ID: <2600872.Lt9SDvczpP@suse>
In-Reply-To: <20221016150656.5803-1-fmdefrancesco@gmail.com>
References: <20221016150656.5803-1-fmdefrancesco@gmail.com>
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On domenica 16 ottobre 2022 17:06:56 CET Fabio M. De Francesco wrote:
> The use of kmap() and kmap_atomic() are being deprecated in favor of
> kmap_local_page().
>=20
> There are two main problems with kmap(): (1) It comes with an overhead as
> the mapping space is restricted and protected by a global lock for
> synchronization and (2) it also requires global TLB invalidation when the
> kmap=E2=80=99s pool wraps and it might block when the mapping space is fu=
lly
> utilized until a slot becomes available.
>=20
> With kmap_local_page() the mappings are per thread, CPU local, can take
> page faults, and can be called from any context (including interrupts).
> It is faster than kmap() in kernels with HIGHMEM enabled. Furthermore,
> the tasks can be preempted and, when they are scheduled to run again, the
> kernel virtual addresses are restored and still valid.
>=20
> Since its use in fs/aio.c is safe everywhere, it should be preferred.
>=20
> Therefore, replace kmap() and kmap_atomic() with kmap_local_page() in
> fs/aio.c.
>=20
> Tested with xfstests on a QEMU/KVM x86_32 VM, 6GB RAM, booting a kernel
> with HIGHMEM64GB enabled.
>=20
> Cc: "Venkataramanan, Anirudh" <anirudh.venkataramanan@intel.com>
> Suggested-by: Ira Weiny <ira.weiny@intel.com>
> Reviewed-by: Ira Weiny <ira.weiny@intel.com>
> Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
> ---
>=20
> I've tested with "./check -g aio". The tests in this group fail 3/26
> times, with and without my patch. Therefore, these changes don't introduce
> further errors. I'm not aware of any further tests I may run, so that
> any suggestions would be precious and much appreciated :-)
>=20
> I'm resending this patch because some recipients were missing in the
> previous submissions. In the meantime I'm also adding some more informati=
on
> in the commit message. There are no changes in the code.
>=20
>  fs/aio.c | 32 ++++++++++++++++----------------
>  1 file changed, 16 insertions(+), 16 deletions(-)
>=20
> diff --git a/fs/aio.c b/fs/aio.c
> index 3c249b938632..343fea0c6d1a 100644
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -567,7 +567,7 @@ static int aio_setup_ring(struct kioctx *ctx, unsigne=
d=20
int
> nr_events) ctx->user_id =3D ctx->mmap_base;
>  	ctx->nr_events =3D nr_events; /* trusted copy */
>=20
> -	ring =3D kmap_atomic(ctx->ring_pages[0]);
> +	ring =3D kmap_local_page(ctx->ring_pages[0]);
>  	ring->nr =3D nr_events;	/* user copy */
>  	ring->id =3D ~0U;
>  	ring->head =3D ring->tail =3D 0;
> @@ -575,7 +575,7 @@ static int aio_setup_ring(struct kioctx *ctx, unsigne=
d=20
int
> nr_events) ring->compat_features =3D AIO_RING_COMPAT_FEATURES;
>  	ring->incompat_features =3D AIO_RING_INCOMPAT_FEATURES;
>  	ring->header_length =3D sizeof(struct aio_ring);
> -	kunmap_atomic(ring);
> +	kunmap_local(ring);
>  	flush_dcache_page(ctx->ring_pages[0]);
>=20
>  	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 =3D kmap_atomic(ctx-
>ring_pages[0]);
> +					ring =3D kmap_local_page(ctx-
>ring_pages[0]);
>  					ring->id =3D ctx->id;
> -					kunmap_atomic(ring);
> +					kunmap_local(ring);
>  					return 0;
>  				}
>=20
> @@ -1024,9 +1024,9 @@ static void user_refill_reqs_available(struct kioctx
> *ctx) * against ctx->completed_events below will make sure we do the
>  		 * safe/right thing.
>  		 */
> -		ring =3D kmap_atomic(ctx->ring_pages[0]);
> +		ring =3D kmap_local_page(ctx->ring_pages[0]);
>  		head =3D ring->head;
> -		kunmap_atomic(ring);
> +		kunmap_local(ring);
>=20
>  		refill_reqs_available(ctx, head, ctx->tail);
>  	}
> @@ -1132,12 +1132,12 @@ static void aio_complete(struct aio_kiocb *iocb)
>  	if (++tail >=3D ctx->nr_events)
>  		tail =3D 0;
>=20
> -	ev_page =3D kmap_atomic(ctx->ring_pages[pos / AIO_EVENTS_PER_PAGE]);
> +	ev_page =3D kmap_local_page(ctx->ring_pages[pos /=20
AIO_EVENTS_PER_PAGE]);
>  	event =3D ev_page + pos % AIO_EVENTS_PER_PAGE;
>=20
>  	*event =3D iocb->ki_res;
>=20
> -	kunmap_atomic(ev_page);
> +	kunmap_local(ev_page);
>  	flush_dcache_page(ctx->ring_pages[pos / AIO_EVENTS_PER_PAGE]);
>=20
>  	pr_debug("%p[%u]: %p: %p %Lx %Lx %Lx\n", ctx, tail, iocb,
> @@ -1151,10 +1151,10 @@ static void aio_complete(struct aio_kiocb *iocb)
>=20
>  	ctx->tail =3D tail;
>=20
> -	ring =3D kmap_atomic(ctx->ring_pages[0]);
> +	ring =3D kmap_local_page(ctx->ring_pages[0]);
>  	head =3D ring->head;
>  	ring->tail =3D tail;
> -	kunmap_atomic(ring);
> +	kunmap_local(ring);
>  	flush_dcache_page(ctx->ring_pages[0]);
>=20
>  	ctx->completed_events++;
> @@ -1214,10 +1214,10 @@ static long aio_read_events_ring(struct kioctx *c=
tx,
>  	mutex_lock(&ctx->ring_lock);
>=20
>  	/* Access to ->ring_pages here is protected by ctx->ring_lock. */
> -	ring =3D kmap_atomic(ctx->ring_pages[0]);
> +	ring =3D kmap_local_page(ctx->ring_pages[0]);
>  	head =3D ring->head;
>  	tail =3D ring->tail;
> -	kunmap_atomic(ring);
> +	kunmap_local(ring);
>=20
>  	/*
>  	 * Ensure that once we've read the current tail pointer, that
> @@ -1249,10 +1249,10 @@ static long aio_read_events_ring(struct kioctx *c=
tx,
>  		avail =3D min(avail, nr - ret);
>  		avail =3D min_t(long, avail, AIO_EVENTS_PER_PAGE - pos);
>=20
> -		ev =3D kmap(page);
> +		ev =3D kmap_local_page(page);
>  		copy_ret =3D copy_to_user(event + ret, ev + pos,
>  					sizeof(*ev) * avail);
> -		kunmap(page);
> +		kunmap_local(ev);
>=20
>  		if (unlikely(copy_ret)) {
>  			ret =3D -EFAULT;
> @@ -1264,9 +1264,9 @@ static long aio_read_events_ring(struct kioctx *ctx,
>  		head %=3D ctx->nr_events;
>  	}
>=20
> -	ring =3D kmap_atomic(ctx->ring_pages[0]);
> +	ring =3D kmap_local_page(ctx->ring_pages[0]);
>  	ring->head =3D head;
> -	kunmap_atomic(ring);
> +	kunmap_local(ring);
>  	flush_dcache_page(ctx->ring_pages[0]);
>=20
>  	pr_debug("%li  h%u t%u\n", ret, head, tail);
> --
> 2.36.1

Al, Benjamin,

I'm sending a gentle ping for this old patch too (and thanking Al again for=
=20
pointing me out how fs/ufs and fs/sysv conversions must be reworked and=20
mistakes fixed).

About this I've had Ira's and Jeff's "Reviewed-by:" tags. I also responded =
in=20
this thread to a couple of objections from Jeff which were regarding some=20
ambiguities in the commit message.

Please let me know if here too there are mistakes which must be fixed and c=
ode=20
to be reworked.

I'm currently just a little more than a pure hobbyist, therefore please be=
=20
patient for the time it takes because until mid February 2023 I'll only be=
=20
able to work few hours per weeks using my spare time.=20

I'm looking forward to hearing from you.

Regards,

=46abio