From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758163AbaD2PyN (ORCPT ); Tue, 29 Apr 2014 11:54:13 -0400 Received: from mx1.redhat.com ([209.132.183.28]:32187 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754685AbaD2PyL (ORCPT ); Tue, 29 Apr 2014 11:54:11 -0400 Date: Tue, 29 Apr 2014 17:53:59 +0200 From: Maurizio Lombardi To: viro@zeniv.linux.org.uk Cc: akpm@linux-foundation.org, linux-fsdevel@vger.kernel.org, JBottomley@parallels.com, hch@lst.de, linux-scsi@vger.kernel.org, kmo@daterainc.com, linux-kernel@vger.kernel.org, m.lombardi85@gmail.com Subject: Re: [PATCH] bio: modify __bio_add_page() to accept pages that don't start a new segment Message-ID: <20140429155359.GA10802@dhcp-27-189.brq.redhat.com> References: <1398783498-19814-1-git-send-email-mlombard@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <1398783498-19814-1-git-send-email-mlombard@redhat.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Sorry I did a mistake in this patch: on failure I should restore the original value of bi_phys_segments. I'm going to send a new version. Maurizio Lombardi On Tue, Apr 29, 2014 at 04:58:18PM +0200, Maurizio Lombardi wrote: > The original behaviour is to refuse to add a new page if the maximum number > of segments has been reached, regardless of the fact the page we are > going to add can be merged into the last segment or not. > > Unfortunately, when the system runs under heavy memory fragmentation conditions, > a driver may try to add multiple pages to the last segment. > The original code won't accept them and EBUSY will be reported to > userspace. > > This patch modifies the function so it refuses to add a page > only in case the latter starts a new segment and the maximum number > of segments has already been reached. > > The bug can be easily reproduced with the st driver: > > 1) set CONFIG_SCSI_MPT2SAS_MAX_SGE or CONFIG_SCSI_MPT3SAS_MAX_SGE to 16 > 2) modprobe st buffer_kbs=1024 > 3) #dd if=/dev/zero of=/dev/st0 bs=1M count=10 > dd: error writing ‘/dev/st0’: Device or resource busy > > Signed-off-by: Maurizio Lombardi > --- > fs/bio.c | 50 ++++++++++++++++++++++++++++---------------------- > 1 file changed, 28 insertions(+), 22 deletions(-) > > diff --git a/fs/bio.c b/fs/bio.c > index 6f0362b..9a3a0b1 100644 > --- a/fs/bio.c > +++ b/fs/bio.c > @@ -750,29 +750,31 @@ static int __bio_add_page(struct request_queue *q, struct bio *bio, struct page > return 0; > > /* > - * we might lose a segment or two here, but rather that than > - * make this too complex. > + * setup the new entry, we might clear it again later if we > + * cannot add the page > + */ > + bvec = &bio->bi_io_vec[bio->bi_vcnt]; > + bvec->bv_page = page; > + bvec->bv_len = len; > + bvec->bv_offset = offset; > + bio->bi_vcnt++; > + bio->bi_phys_segments++; > + > + /* > + * Perform a recount if the number of segments is greater > + * than queue_max_segments(q). > */ > > - while (bio->bi_phys_segments >= queue_max_segments(q)) { > + while (bio->bi_phys_segments > queue_max_segments(q)) { > > if (retried_segments) > - return 0; > + goto failed; > > retried_segments = 1; > blk_recount_segments(q, bio); > } > > /* > - * setup the new entry, we might clear it again later if we > - * cannot add the page > - */ > - bvec = &bio->bi_io_vec[bio->bi_vcnt]; > - bvec->bv_page = page; > - bvec->bv_len = len; > - bvec->bv_offset = offset; > - > - /* > * if queue has other restrictions (eg varying max sector size > * depending on offset), it can specify a merge_bvec_fn in the > * queue to get further control > @@ -789,23 +791,27 @@ static int __bio_add_page(struct request_queue *q, struct bio *bio, struct page > * merge_bvec_fn() returns number of bytes it can accept > * at this offset > */ > - if (q->merge_bvec_fn(q, &bvm, bvec) < bvec->bv_len) { > - bvec->bv_page = NULL; > - bvec->bv_len = 0; > - bvec->bv_offset = 0; > - return 0; > - } > + if (q->merge_bvec_fn(q, &bvm, bvec) < bvec->bv_len) > + goto failed; > } > > /* If we may be able to merge these biovecs, force a recount */ > - if (bio->bi_vcnt && (BIOVEC_PHYS_MERGEABLE(bvec-1, bvec))) > + if (bio->bi_vcnt > 1 && (BIOVEC_PHYS_MERGEABLE(bvec-1, bvec))) > bio->bi_flags &= ~(1 << BIO_SEG_VALID); > > - bio->bi_vcnt++; > - bio->bi_phys_segments++; > done: > bio->bi_iter.bi_size += len; > return len; > + > + failed: > + bvec->bv_page = NULL; > + bvec->bv_len = 0; > + bvec->bv_offset = 0; > + bio->bi_vcnt--; > + if (!retried_segments) > + bio->bi_phys_segments--; > + > + return 0; > } > > /** > -- > Maurizio Lombardi > > -- > To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html