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[172.10.233.147]) by smtp.gmail.com with ESMTPSA id 3f1490d57ef6-e8b7afcd1d2sm3729461276.45.2025.07.15.13.03.55 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 15 Jul 2025 13:03:57 -0700 (PDT) Date: Tue, 15 Jul 2025 13:03:40 -0700 (PDT) From: Hugh Dickins To: Lorenzo Stoakes , Andrew Morton cc: Baolin Wang , Matthew Wilcox , hughd@google.com, david@redhat.com, ziy@nvidia.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, vbabka@suse.cz, rppt@kernel.org, surenb@google.com, mhocko@suse.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] mm: fault in complete folios instead of individual pages for tmpfs In-Reply-To: Message-ID: <3bf50873-4d1b-a7c7-112e-3a17ac16077f@google.com> References: <440940e78aeb7430c5cc8b6d2088ae98265b9809.1751599072.git.baolin.wang@linux.alibaba.com> <20250704151858.73d35a24b4c2f53bdb0c1b85@linux-foundation.org> <4c055849-d7dd-4b9f-9666-fcb0bccf8681@linux.alibaba.com> <007c4a94-c94a-418e-9907-7510422f8ca4@lucifer.local> <23f1c3ab-16ca-41db-b008-22448d9e08f2@linux.alibaba.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII On Wed, 9 Jul 2025, Lorenzo Stoakes wrote: > On Tue, Jul 08, 2025 at 03:53:56PM +0800, Baolin Wang wrote: > > On 2025/7/7 21:33, Lorenzo Stoakes wrote: > > > On Sun, Jul 06, 2025 at 10:02:35AM +0800, Baolin Wang wrote: > > > > On 2025/7/5 06:18, Andrew Morton wrote: > > > > > On Fri, 4 Jul 2025 11:19:26 +0800 Baolin Wang wrote: > > > > > > > > > > > After commit acd7ccb284b8 ("mm: shmem: add large folio support for tmpfs"), > > > > > > tmpfs can also support large folio allocation (not just PMD-sized large > > > > > > folios). > > > > > > > > > > > > However, when accessing tmpfs via mmap(), although tmpfs supports large folios, > > > > > > we still establish mappings at the base page granularity, which is unreasonable. > > > > > > > > > > > > We can map multiple consecutive pages of a tmpfs folios at once according to > > > > > > the size of the large folio. On one hand, this can reduce the overhead of page > > > > > > faults; on the other hand, it can leverage hardware architecture optimizations > > > > > > to reduce TLB misses, such as contiguous PTEs on the ARM architecture. > > > > > > > > > > > > Moreover, tmpfs mount will use the 'huge=' option to control large folio > > > > > > allocation explicitly. So it can be understood that the process's RSS statistics > > > > > > might increase, and I think this will not cause any obvious effects for users. > > > > > > > > > > > > Performance test: > > > > > > I created a 1G tmpfs file, populated with 64K large folios, and write-accessed it > > > > > > sequentially via mmap(). I observed a significant performance improvement: > > > > > > > > > > That doesn't sound like a crazy thing to do. > > > > > > > > > > > Before the patch: > > > > > > real 0m0.158s > > > > > > user 0m0.008s > > > > > > sys 0m0.150s > > > > > > > > > > > > After the patch: > > > > > > real 0m0.021s > > > > > > user 0m0.004s > > > > > > sys 0m0.017s > > > > > > > > > > And look at that. > > > > > > > > > > > diff --git a/mm/memory.c b/mm/memory.c > > > > > > index 0f9b32a20e5b..9944380e947d 100644 > > > > > > --- a/mm/memory.c > > > > > > +++ b/mm/memory.c > > > > > > @@ -5383,10 +5383,10 @@ vm_fault_t finish_fault(struct vm_fault *vmf) > > > > > > /* > > > > > > * Using per-page fault to maintain the uffd semantics, and same > > > > > > - * approach also applies to non-anonymous-shmem faults to avoid > > > > > > + * approach also applies to non shmem/tmpfs faults to avoid > > > > > > * inflating the RSS of the process. > > > > > > */ > > > > > > - if (!vma_is_anon_shmem(vma) || unlikely(userfaultfd_armed(vma)) || > > > > > > + if (!vma_is_shmem(vma) || unlikely(userfaultfd_armed(vma)) || > > > > > > unlikely(needs_fallback)) { > > > > > > nr_pages = 1; > > > > > > } else if (nr_pages > 1) { > > > > > > > > > > and that's it? > > > > > > > > > > I'm itching to get this into -stable, really. What LTS user wouldn't > > > > > want this? > > > > > > > > This is an improvement rather than a bugfix, so I don't think it needs to go > > > > into LTS. > > > > > > > > Could it be viewed as correcting an oversight in > > > > > acd7ccb284b8? > > > > > > > > Yes, I should have added this optimization in the series of the commit > > > > acd7ccb284b8. But obviously, I missed this :(. > > > > > > Buuut if this was an oversight for that patch that causes an unnecessary > > > perf degradation, surely this should have fixes tag + cc stable no? > > > > IMO, this commit acd7ccb284b8 won't cause perf degradation, instead it is > > used to introduce a new feature, while the current patch is a further > > reasonable optimization. As I mentioned, this is an improvement, not a > > bugfix or a patch to address performance regression. > > 4Well :) you say yourself it was an oversight, and it very clearly has a perf > _impact_, which if you compare backwards to acd7ccb284b8 is a degradation, but I > get your point. > > However, since you say 'oversight' this seems to me that you really meant to > have included it but hadn't noticed, and additionally, since it just seems to be > an unequivical good - let's maybe flip this round - why NOT backport it to > stable? I strongly agree with Baolin: this patch is good, thank you, but it is a performance improvement, a new feature, not a candidate for the stable tree. I'm surprised anyone thinks otherwise: Andrew, please delete that stable tag before advancing the patch from mm-unstable to mm-stable. And the Fixee went into 6.14, so it couldn't go to 6.12 LTS anyway. An unequivocal good? I'm not so sure. I expect it ought to be limited, by fault_around_bytes (or suchlike). If I understand all the mTHP versus large folio versus PMD-huge handling correctly (and of course I do not, I'm still weeks if not months away from understanding most of it), the old vma_is_anon_shmem() case would be limited by the shmem mTHP tunables, and one can reasonably argue that they would already take fault_around_bytes-like considerations into account; but the newly added file-written cases are governed by huge= mount options intended for PMD-size, but (currently) permitting all lesser orders. I don't think that mounting a tmpfs huge=always implies that mapping 256 PTEs for one fault is necessarily a good strategy. But looking in the opposite direction, why is there now a vma_is_shmem() check there in finish_fault() at all? If major filesystems are using large folios, why aren't they also allowed to benefit from mapping multiple PTEs at once (in this shared-writable case which the existing fault-around does not cover - I presume to avoid write amplification, but that's not an issue when the folio is large already). It's fine to advance cautiously, keeping this to shmem in coming release; but I think it should be extended soon (without any backport to stable), and consideration given to limiting it. Hugh