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Thu, 11 Jun 2020 18:20:29 +0000 Date: Thu, 11 Jun 2020 11:20:29 -0700 From: Matthew Wilcox To: linux-fsdevel@vger.kernel.org Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v6 20/51] block: Add bio_for_each_thp_segment_all Message-ID: <20200611182029.GC8681@bombadil.infradead.org> References: <20200610201345.13273-1-willy@infradead.org> <20200610201345.13273-21-willy@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200610201345.13273-21-willy@infradead.org> X-Rspamd-Queue-Id: A944E18016847 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam03 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, Jun 10, 2020 at 01:13:14PM -0700, Matthew Wilcox wrote: > +static inline void bvec_thp_advance(const struct bio_vec *bvec, > + struct bvec_iter_all *iter_all) > +{ > + struct bio_vec *bv = &iter_all->bv; > + unsigned int page_size = thp_size(bvec->bv_page); > + > + if (iter_all->done) { > + bv->bv_page += thp_nr_pages(bv->bv_page); > + bv->bv_offset = 0; > + } else { > + BUG_ON(bvec->bv_offset >= page_size); > + bv->bv_page = bvec->bv_page; > + bv->bv_offset = bvec->bv_offset & (page_size - 1); > + } > + bv->bv_len = min(page_size - bv->bv_offset, > + bvec->bv_len - iter_all->done); > + iter_all->done += bv->bv_len; > + > + if (iter_all->done == bvec->bv_len) { > + iter_all->idx++; > + iter_all->done = 0; > + } > +} If, for example, we have an order-2 page followed by two order-0 pages (thanks, generic/127!) in the bvec, we'll end up skipping the third page because we calculate the size based on bvec->bv_page instead of bv->bv_page. +++ b/include/linux/bvec.h @@ -166,15 +166,19 @@ static inline void bvec_thp_advance(const struct bio_vec *bvec, struct bvec_iter_all *iter_all) { struct bio_vec *bv = &iter_all->bv; - unsigned int page_size = thp_size(bvec->bv_page); + unsigned int page_size; if (iter_all->done) { bv->bv_page += thp_nr_pages(bv->bv_page); + page_size = thp_size(bv->bv_page); bv->bv_offset = 0; } else { - BUG_ON(bvec->bv_offset >= page_size); - bv->bv_page = bvec->bv_page; - bv->bv_offset = bvec->bv_offset & (page_size - 1); + bv->bv_page = thp_head(bvec->bv_page + + (bvec->bv_offset >> PAGE_SHIFT)); + page_size = thp_size(bv->bv_page); + bv->bv_offset = bvec->bv_offset - + (bv->bv_page - bvec->bv_page) * PAGE_SIZE; + BUG_ON(bv->bv_offset >= page_size); } bv->bv_len = min(page_size - bv->bv_offset, bvec->bv_len - iter_all->done); The previous code also wasn't handling the case fixed in 6bedf00e55e5 where a split bio might end up splitting a bvec. That BUG_ON can probably come out after a few months of testing.