Re: [RESEND PATCH] fs/aio: Replace kmap{,_atomic}() with kmap_local_page()

["Fabio M. De Francesco" <fmdefrancesco@gmail.com>]

On giovedì 1 dicembre 2022 15:29:17 CET Fabio M. De Francesco wrote:
> 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().
> > 
> > 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’s pool wraps and it might block when the mapping space is fully
> > utilized until a slot becomes available.
> > 
> > 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.
> > 
> > Since its use in fs/aio.c is safe everywhere, it should be preferred.
> > 
> > Therefore, replace kmap() and kmap_atomic() with kmap_local_page() in
> > fs/aio.c.
> > 
> > Tested with xfstests on a QEMU/KVM x86_32 VM, 6GB RAM, booting a kernel
> > with HIGHMEM64GB enabled.
> > 
> > Cc: "Venkataramanan, Anirudh" <anirudh.venkataramanan@intel.com>
> > Suggested-by: Ira Weiny <ira.weiny@intel.com>
> > Reviewed-by: Ira Weiny <ira.weiny@intel.com>
> 
> Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
> 
> > Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
> > ---
> 
> I'm sorry to resend again. Last time I forgot to forward the "Reviewed-by:"
> tag from Jeff (thanks!).
> 
> > 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 :-)
> > 
> > I'm resending this patch because some recipients were missing in the
> > previous submissions. In the meantime I'm also adding some more 
information
> > in the commit message. There are no changes in the code.
> > 
> >  fs/aio.c | 32 ++++++++++++++++----------------
> >  1 file changed, 16 insertions(+), 16 deletions(-)
> > 
> > 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, unsigned
> 
> int
> 
> > nr_events) ctx->user_id = ctx->mmap_base;
> > 
> >  	ctx->nr_events = nr_events; /* trusted copy */
> > 
> > -	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]);
> >  	
> >  	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);
> > 
> >  					return 0;
> >  				
> >  				}
> > 
> > @@ -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 = kmap_atomic(ctx->ring_pages[0]);
> > +		ring = kmap_local_page(ctx->ring_pages[0]);
> > 
> >  		head = ring->head;
> > 
> > -		kunmap_atomic(ring);
> > +		kunmap_local(ring);
> > 
> >  		refill_reqs_available(ctx, head, ctx->tail);
> >  	
> >  	}
> > 
> > @@ -1132,12 +1132,12 @@ static void aio_complete(struct aio_kiocb *iocb)
> > 
> >  	if (++tail >= ctx->nr_events)
> >  	
> >  		tail = 0;
> > 
> > -	ev_page = kmap_atomic(ctx->ring_pages[pos / AIO_EVENTS_PER_PAGE]);
> > +	ev_page = kmap_local_page(ctx->ring_pages[pos /
> 
> AIO_EVENTS_PER_PAGE]);
> 
> >  	event = ev_page + pos % AIO_EVENTS_PER_PAGE;
> >  	
> >  	*event = iocb->ki_res;
> > 
> > -	kunmap_atomic(ev_page);
> > +	kunmap_local(ev_page);
> > 
> >  	flush_dcache_page(ctx->ring_pages[pos / AIO_EVENTS_PER_PAGE]);
> >  	
> >  	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)
> > 
> >  	ctx->tail = tail;
> > 
> > -	ring = kmap_atomic(ctx->ring_pages[0]);
> > +	ring = kmap_local_page(ctx->ring_pages[0]);
> > 
> >  	head = ring->head;
> >  	ring->tail = tail;
> > 
> > -	kunmap_atomic(ring);
> > +	kunmap_local(ring);
> > 
> >  	flush_dcache_page(ctx->ring_pages[0]);
> >  	
> >  	ctx->completed_events++;
> > 
> > @@ -1214,10 +1214,10 @@ static long aio_read_events_ring(struct kioctx 
*ctx,
> > 
> >  	mutex_lock(&ctx->ring_lock);
> >  	
> >  	/* Access to ->ring_pages here is protected by ctx->ring_lock. */
> > 
> > -	ring = kmap_atomic(ctx->ring_pages[0]);
> > +	ring = kmap_local_page(ctx->ring_pages[0]);
> > 
> >  	head = ring->head;
> >  	tail = ring->tail;
> > 
> > -	kunmap_atomic(ring);
> > +	kunmap_local(ring);
> > 
> >  	/*
> >  	
> >  	 * Ensure that once we've read the current tail pointer, that
> > 
> > @@ -1249,10 +1249,10 @@ static long aio_read_events_ring(struct kioctx 
*ctx,
> > 
> >  		avail = min(avail, nr - ret);
> >  		avail = min_t(long, avail, AIO_EVENTS_PER_PAGE - pos);
> > 
> > -		ev = kmap(page);
> > +		ev = kmap_local_page(page);
> > 
> >  		copy_ret = copy_to_user(event + ret, ev + pos,
> >  		
> >  					sizeof(*ev) * avail);
> > 
> > -		kunmap(page);
> > +		kunmap_local(ev);
> > 
> >  		if (unlikely(copy_ret)) {
> >  		
> >  			ret = -EFAULT;
> > 
> > @@ -1264,9 +1264,9 @@ static long aio_read_events_ring(struct kioctx *ctx,
> > 
> >  		head %= ctx->nr_events;
> >  	
> >  	}
> > 
> > -	ring = kmap_atomic(ctx->ring_pages[0]);
> > +	ring = kmap_local_page(ctx->ring_pages[0]);
> > 
> >  	ring->head = head;
> > 
> > -	kunmap_atomic(ring);
> > +	kunmap_local(ring);
> > 
> >  	flush_dcache_page(ctx->ring_pages[0]);
> >  	
> >  	pr_debug("%li  h%u t%u\n", ret, head, tail);
> > 
> > --
> > 2.36.1

Please disregard this patch because I just sent a v2 with some additional 
information in the commit message and added Jeff's "Reviewed-by" tag.

Thanks,

Fabio

[1] https://lore.kernel.org/lkml/20230109175629.9482-1-fmdefrancesco@gmail.com/