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b=dFQXMKv/KNYEB64BV/AgTJB+aPpPZ06zWHIP9CA0fci5FQuiHZbD1okMuxnsmq1Ni vaL/sLkSg6qa0Fb9WxmYe0X45WBE2uFw23olR+2xfsEDppyuDGiA0kbetMqvzfAQ2X uFzTK2TRFvKNBWG/Nk+zvi7zc55FhOqmf2aKduLIr5aTyhHu1RT0zx6q6opC225jVA kmDMNRFXa4+XuYPAe4sZ2T+g52ibSaTmbSSscfYAjn8ECpGQqMAoBRU0Iq9LwSXfKS jbw20kN+vz8Yz9FO0GzTIsxN3CjzvRFJY5udy6nBeGBpv+2a2PZiT6gIfIH9jDO7+R SIH2APtntKorA== Date: Thu, 10 Jul 2025 10:29:22 -0600 From: Keith Busch To: Mike Snitzer Cc: Ming Lei , Jens Axboe , Jeff Layton , Chuck Lever , NeilBrown , Olga Kornievskaia , Dai Ngo , Tom Talpey , Trond Myklebust , Anna Schumaker , linux-nfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, hch@infradead.org, linux-block@vger.kernel.org Subject: Re: [RFC PATCH v2 4/8] lib/iov_iter: remove piecewise bvec length checking in iov_iter_aligned_bvec Message-ID: References: <20250708160619.64800-1-snitzer@kernel.org> <20250708160619.64800-5-snitzer@kernel.org> <5819d6c5bb194613a14d2dcf05605e701683ba49.camel@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 2C63516000A X-Stat-Signature: absme983b41y31xc14fyxqa5issxwe4z X-Rspam-User: X-HE-Tag: 1752164967-138849 X-HE-Meta: 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 n7vwvlgT iL5cTJm8wNc7Cclrr0XC4XIvgqr76eiiLCWb+gQe1kE/E7QU4PtUUvDyxOuxTrfDz+sLKGrfiJegIss2ARJy35qx+TOp/U7GX5UGYOXIEtRJaXdvi3s7QoegJAp14hTDhF/P8CAEax4qgyJMOvhFXbbONQ8jkV4F3vyzCLjbyJvtI8J2t1n/tmvAtOPYE7DaUPllmzHG1xtEvOi82F5dld8ZnAFe3ZP5Ts9pe+CWds3m96dBrs1Z0znWyyrN2yKs68hjDHZIW6i6DyzCWjrIB9InN67/1l6gFZFsYkozBPzQGrZzy8d3GJjv6h5j0RL9JKCjd 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: List-Subscribe: List-Unsubscribe: On Thu, Jul 10, 2025 at 12:12:29PM -0400, Mike Snitzer wrote: > On Thu, Jul 10, 2025 at 08:48:04AM -0600, Keith Busch wrote: > > On Thu, Jul 10, 2025 at 09:52:53AM -0400, Jeff Layton wrote: > > > On Tue, 2025-07-08 at 12:06 -0400, Mike Snitzer wrote: > > > > iov_iter_aligned_bvec() is strictly checking alignment of each element > > > > of the bvec to arrive at whether the bvec is aligned relative to > > > > dma_alignment and on-disk alignment. Checking each element > > > > individually results in disallowing a bvec that in aggregate is > > > > perfectly aligned relative to the provided @len_mask. > > > > > > > > Relax the on-disk alignment checking such that it is done on the full > > > > extent described by the bvec but still do piecewise checking of the > > > > dma_alignment for each bvec's bv_offset. > > > > > > > > This allows for NFS's WRITE payload to be issued using O_DIRECT as > > > > long as the bvec created with xdr_buf_to_bvec() is composed of pages > > > > that respect the underlying device's dma_alignment (@addr_mask) and > > > > the overall contiguous on-disk extent is aligned relative to the > > > > logical_block_size (@len_mask). > > > > > > > > Signed-off-by: Mike Snitzer > > > > --- > > > > lib/iov_iter.c | 5 +++-- > > > > 1 file changed, 3 insertions(+), 2 deletions(-) > > > > > > > > diff --git a/lib/iov_iter.c b/lib/iov_iter.c > > > > index bdb37d572e97..b2ae482b8a1d 100644 > > > > --- a/lib/iov_iter.c > > > > +++ b/lib/iov_iter.c > > > > @@ -819,13 +819,14 @@ static bool iov_iter_aligned_bvec(const struct iov_iter *i, unsigned addr_mask, > > > > unsigned skip = i->iov_offset; > > > > size_t size = i->count; > > > > > > > > + if (size & len_mask) > > > > + return false; > > > > + > > > > do { > > > > size_t len = bvec->bv_len; > > > > > > > > if (len > size) > > > > len = size; > > > > - if (len & len_mask) > > > > - return false; > > > > if ((unsigned long)(bvec->bv_offset + skip) & addr_mask) > > > > return false; > > > > > > > > > > cc'ing Keith too since he wrote this helper originally. > > > > Thanks. > > > > There's a comment in __bio_iov_iter_get_pages that says it expects each > > vector to be a multiple of the block size. That makes it easier to > > slit when needed, and this patch would allow vectors that break the > > current assumption when calculating the "trim" value. > > Thanks for the pointer, that high-level bio code is being too > restrictive. > > But not seeing any issues with the trim calculation itself, 'trim' is > the number of bytes that are past the last logical_block_size aligned > boundary. And then iov_iter_revert() will rollback the iov such that > it doesn't include those. Then size is reduced by trim bytes. The trim calculation assumes the current bi_size is already a block size multiple, but it may not be with your propsal. So the trim bytes needs to take into account the existing bi_size to know how much to trim off to arrive at a proper total bi_size instead of assuming we can append a block sized multiple carved out the current iov. > All said, in practice I haven't had any issues with this patch. But > it could just be I don't have the stars aligned to test the case that > might have problems. If you know of such a case I'd welcome > suggestions. It might be a little harder with iter_bvec, but you also mentioned doing the same for iter_iovec too, which I think should be pretty easy to cause a problem for nvme: just submit an O_DIRECT read or write with individual iovec sizes that are not block size granularities.