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 C4AD5221FBB for ; Thu, 28 Aug 2025 16:36:22 +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=1756398983; cv=none; b=hr6Di91Wj8P1dsEchu237J3RAnSgjrac5Jd8wlEGR/XxGT6Wsdzc4w/18poMOyyJKs8fJrumZpH8JqmAL2n0aC91G1jibJ06+XMDDg4/bEwJqzmZacJjawPgpyNHOXSKbrerYdbSW3S6Gj37SjcmN5dipCrkh9kAwsKeR+7xJFc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1756398983; c=relaxed/simple; bh=hQEp3NTqU4Sr6sX/JJj6YtJSXoTYyv01gQV8HMrpD48=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=PD22ezUqWODRS7MF3HkqrY2l8+Cwqe0MD8wbcEUfK0oRazSn+LOvIraz/Ez/iis+86aLcJG144LXdNluDCeAPHG6SxbdDHu8FyDw+5L4y2ilRAVSkwYVwTj0DgzQeLRT3+X/oXK4eG5YUyRXxww2lOrdT3ypNbZlW0qT0qDQa6g= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=CEg60fZ2; 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="CEg60fZ2" Received: by smtp.kernel.org (Postfix) with ESMTPSA id EDED4C4CEEB; Thu, 28 Aug 2025 16:36:21 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1756398982; bh=hQEp3NTqU4Sr6sX/JJj6YtJSXoTYyv01gQV8HMrpD48=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=CEg60fZ2YOErIuQkuSPaqldT+uBGK2opDkZMgXkp8Tw+8a4VrSaRzWw8sY/QjsWrI eOmiuFirjbl9ajhkdehNJIdxDU9yb9QvxFAWlL5aaWJ9o+rLVj+0cGV7LeRKnplk4K FMwPG76wrbq3L1//OCJIcr+uy+Bx/Tx/TyVs5Wu3efspkASlhN+13204bdvWH6UZky lfXV3RAqFpkGFoXH24mek6vEHBxJi9oUXWbwqFQ3lQyeuv/uR7wfoAJ1uFUfByBNlK pT5IAIFiXLXyHBEIVlwoN/wa0HmjDPCKNT8kUmd81GgQ9S0RlqT/TsmslIqhk5TB2C aZYedK7ISjjJA== Message-ID: <09fa0bb261508aa9da87d2e664bae064f2232eaf.camel@kernel.org> Subject: Re: [PATCH v8 5/7] NFSD: issue READs using O_DIRECT even if IO is misaligned From: Jeff Layton To: Mike Snitzer , Chuck Lever Cc: linux-nfs@vger.kernel.org Date: Thu, 28 Aug 2025 12:36:20 -0400 In-Reply-To: References: <20250826185718.5593-1-snitzer@kernel.org> <20250826185718.5593-6-snitzer@kernel.org> <6f5516a5-1954-4f77-8a07-dacba1fb570c@oracle.com> <09eca412-b6e3-4011-b7dd-3a452eae6489@oracle.com> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV 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Thu, 2025-08-28 at 04:09 -0400, Mike Snitzer wrote: > On Wed, Aug 27, 2025 at 09:57:39PM -0400, Chuck Lever wrote: > > On 8/27/25 7:15 PM, Mike Snitzer wrote: > > > On Wed, Aug 27, 2025 at 04:56:08PM -0400, Chuck Lever wrote: > > > > On 8/27/25 3:41 PM, Mike Snitzer wrote: > > > > > Is your suggestion to, rather than allocate a disjoint single pag= e, > > > > > borrow the extra page from the end of rq_pages? Just map it into = the > > > > > bvec instead of my extra page? > > > >=20 > > > > Yes, the extra page needs to come from rq_pages. But I don't see wh= y it > > > > should come from the /end/ of rq_pages. > > > >=20 > > > > - Extend the start of the byte range back to make it align with the > > > > file's DIO alignment constraint > > > >=20 > > > > - Extend the end of the byte range forward to make it align with th= e > > > > file's DIO alignment constraint > > >=20 > > > nfsd_analyze_read_dio() does that (start_extra and end_extra). > > >=20 > > > > - Fill in the sink buffer's bvec using pages from rq_pages, as usua= l > > > >=20 > > > > - When the I/O is complete, adjust the offset in the first bvec ent= ry > > > > forward by setting a non-zero page offset, and adjust the returne= d > > > > count downward to match the requested byte count from the client > > >=20 > > > Tried it long ago, such bvec manipulation only works when not using > > > RDMA. When the memory is remote, twiddling a local bvec isn't going > > > to ensure the correct pages have the correct data upon return to the > > > client. > > >=20 > > > RDMA is why the pages must be used in-place, and RDMA is also why > > > the extra page needed by this patch (for use as throwaway front-pad > > > for expanded misaligned DIO READ) must either be allocated _or_ > > > hopefully it can be from rq_pages (after the end of the client > > > requested READ payload). > > >=20 > > > Or am I wrong and simply need to keep learning about NFSD's IO path? > >=20 > > You're wrong, not to put a fine point on it. >=20 > You didn't even understand me.. but firmly believe I'm wrong? >=20 > > There's nothing I can think of in the RDMA or RPC/RDMA protocols that > > mandates that the first page offset must always be zero. Moving data > > at one address on the server to an entirely different address and > > alignment on the client is exactly what RDMA is supposed to do. > >=20 > > It sounds like an implementation omission because the server's upper > > layers have never needed it before now. If TCP already handles it, I'm > > guessing it's going to be straightforward to fix. >=20 > I never said that first page offset must be zero. I said that I > already did what you suggested and it didn't work with RDMA. This is > recall of too many months ago now, but: the client will see the > correct READ payload _except_ IIRC it is offset by whatever front-pad > was added to expand the misaligned DIO; no matter whether > rqstp->rq_bvec updated when IO completes. >=20 > But I'll revisit it again. >=20 > > > > > NFSD using DIO is optional. I thought the point was to get it as = an > > > > > available option so that _others_ could experiment and help categ= orize > > > > > the benefits/pitfalls further? > > > >=20 > > > > Yes, that is the point. But such experiments lose value if there is= no > > > > data collection plan to go with them. > > >=20 > > > Each user runs something they care about performing well and they > > > measure the result. > >=20 > > That assumes the user will continue to use the debug interfaces, and > > the particular implementation you've proposed, for the rest of time. > > And that's not my plan at all. > >=20 > > If we, in the community, cannot reproduce that result, or cannot > > understand what has been measured, or the measurement misses part or > > most of the picture, of what value is that for us to decide whether and > > how to proceed with promoting the mechanism from debug feature to > > something with a long-term support lifetime and a documented ABI-stable > > user interface? >=20 > I'll work to put a finer point on how to reproduce and enumerate the > things to look for (representative flamegraphs showing the issue, > which I already did at last Bakeathon). >=20 > But I have repeatedly offered that the pathological worst case is > client doing sequential write IO of a file that is 3-4x larger than > the NFS server's system memory. >=20 > Large memory systems with 8 or more NVMe devices, fast networks that > allow for huge data ingest capabilities. These are the platforms that > showcase MM's dirty writeback limitions when large sequential IO is > initiated from the NFS client and its able to overrun the NFS server. >=20 > In addition, in general DIO requires significantly less memory and > CPU; so platforms that have more limited resources (and may have > historically struggled) could have a new lease on life if they switch > NFSD from buffered to DIO mode. >=20 > > > Literally the same thing as has been done for anything in Linux since > > > it all started. Nothing unicorn or bespoke here. > >=20 > > So let me ask this another way: What do we need users to measure to giv= e > > us good quality information about the page cache behavior and system > > thrashing behavior you reported? >=20 > IO throughput, CPU and memory usage should be monitored over time. >=20 > > For example: I can enable direct I/O on NFSD, but my workload is mostly > > one or two clients doing kernel builds. The latency of NFS READs goes > > up, but since a kernel build is not I/O bound and the client page cache= s > > hide most of the increase, there is very little to show a measured > > change. > >=20 > > So how should I assess and report the impact of NFSD doing direct I/O? >=20 > Your underwhelming usage isn't what this patchset is meant to help. >=20 > > See -- users are not the only ones who are involved in this experiment; > > and they will need guidance because we're not providing any > > documentation for this feature. >=20 > Users are not created equal. Major companies like Oracle and Meta > _should_ be aware of NFSD's problems with buffered IO. They have > internal and external stakeholders that are power users. >=20 > Jeff, does Meta ever see NFSD struggle to consistently use NVMe > devices? Lumpy performance? Full-blown IO stalls? Lots of NFSD > threads hung in D state? >=20 Yes. We're particularly interested in this work for that reason. A lot of the workload is large, streaming writes at the application layer that are only rarely ever read, and quite a bit later when it does happen. This means that the pagecache is pretty useless. My _guess_ is that DIO will help that significantly, though I do still have some concerns about using buffered I/O for the edges of unaligned WRITEs. > > > > If you would rather make this drive-by, then you'll have to realize > > > > that you are requesting more than simple review from us. You'll hav= e > > > > to be content with the pace at which us overloaded maintainers can = get > > > > to the work. > > >=20 > > > I think I just experienced the mailing-list equivalent of the Detroit > > > definition of "drive-by". Good/bad news: you're a terrible shot. > >=20 > > The term "drive-by contribution" has a well-understood meaning in the > > kernel community. If you are unfamiliar with it, I invite you to review > > the mailing list archives. As always, no-one is shooting at you. If > > anything, the drive-by contribution is aimed at me. >=20 > It is a blatant miscategorization here. That you just doubled down > on it having relevance in this instance is flagrantly wrong. >=20 > Whatever compells you to belittle me and my contributions, just know > it is extremely hard to take. Highly unproductive and unprofessional. >=20 > Boom, done. --=20 Jeff Layton