From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 8382622423A; Thu, 10 Jul 2025 17:23:01 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1752168181; cv=none; b=OC38mianWGPwGUue4pi6j6YZQZKf5eX2IOk92ijVkAQXSu2QFy8IBoDchYwWCpHVg7zGR0oypdDrq3kaCAJKbgj6S0J0RhKggIhPW9Pbke9tLCgj26A9M/AFqQ9cxaqwEd2xUV9x4BzqWuOuquvDoxVsIuePbVmfzR3M3uBKRK8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1752168181; c=relaxed/simple; bh=V255YLFB1n8qZG3lZpwtNT15v0OgXI6hDpqjbbF7/9w=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=bg6BAG3CiAWKreSjrTWgFtoIrUpZjgRuidEgL1pMO353FMyiYcYCHjVZOChfLfgp0sD7iRzg/FlhiAgfGbs0AYDnA+OQXLIXP3tlezVWHnYQ6gj5AGGCSNYagvu6Ee6hUIsMmTdyVe9xPVzToshxe4PlUqTRjbBB0LQcK4o+Vns= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=QKJhFope; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="QKJhFope" Received: by smtp.kernel.org (Postfix) with ESMTPSA id C552AC4CEE3; Thu, 10 Jul 2025 17:23:00 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1752168181; bh=V255YLFB1n8qZG3lZpwtNT15v0OgXI6hDpqjbbF7/9w=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=QKJhFopeKDrpzY+2KvoVXeTP5orLVtemBroS8JmMLRSGXW/E82wWf06sAU3FFfZvL 2Ch64of+GDN5EQC8R8FgRG/oUI0MPV+mCiAwh5RmEVcdwwvCA3479oF5xFEZbx0D/K Vlp60eRAesixtko2acE1HVniyLOxaTPkJvOUmcnUxaRZRhahKsv+lDw85+ugVmaGec 4bhCPpdl4dX6lw3dTHVzKMEj1gXEIaMRp6yEUaGOS2FTu044ccQwCdHCdq5KSM2dOj 2uR3eMK3kBW+mFU9KD6XB2ZfvxFnYZE45umNq2694KBwvo7U22IdiNqjZBA/zZPQVu vm+VbjiRWNuOA== Date: Thu, 10 Jul 2025 13:22:59 -0400 From: Mike Snitzer To: Keith Busch 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> Precedence: bulk X-Mailing-List: linux-block@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Thu, Jul 10, 2025 at 10:29:22AM -0600, Keith Busch wrote: > 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. The trim "calculation" doesn't assume anything, it just lops off whatever is past the end of the last logical_block_size aligned boundary of the requested pages (which is meant to be bi_size). The fact that the trim ever gets anything implies bi_size is *not* always logical_block_size aligned. No? But sure, with my change it opens the door for bvecs with vectors that aren't all logical_block_size aligned. I'll revisit this code, but if you see a way forward to fix __bio_iov_iter_get_pages to cope with my desired iov_iter_aligned_bvec change please don't be shy with a patch ;) > > 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. I made the iter_iovec change yesterday (before I realized I don't actually need it for my NFSD case) and all was fine issuing O_DIRECT IO (via NFSD, so needing the relaxed checking) through to 16 XFS-on-NVMe devices. SO I think the devil will be in the details if NVMe actually cares. Mike