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Thu, 22 May 2025 13:52:11 -0700 (PDT) X-Google-Smtp-Source: AGHT+IFtqLwL4EEjWZoWfka1PUDhJ6Av3HiQxMuLS5exKoiu8NhmzKLHGUSY5WGyjCe+HoDAstPLNg== X-Received: by 2002:a05:6602:160d:b0:867:ddd:bc54 with SMTP id ca18e2360f4ac-86a232cc36cmr868072039f.5.1747947131065; Thu, 22 May 2025 13:52:11 -0700 (PDT) Received: from redhat.com ([38.15.36.11]) by smtp.gmail.com with ESMTPSA id 8926c6da1cb9f-4fbcc48c897sm3331713173.98.2025.05.22.13.52.09 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 22 May 2025 13:52:10 -0700 (PDT) Date: Thu, 22 May 2025 14:52:07 -0600 From: Alex Williamson To: lizhe.67@bytedance.com Cc: david@redhat.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, muchun.song@linux.dev, peterx@redhat.com Subject: Re: [PATCH v4] vfio/type1: optimize vfio_pin_pages_remote() for large folio Message-ID: <20250522145207.01734386.alex.williamson@redhat.com> In-Reply-To: <20250522082524.75076-1-lizhe.67@bytedance.com> References: <81d73c4c-28c4-4fa0-bc71-aef6429e2c31@redhat.com> <20250522082524.75076-1-lizhe.67@bytedance.com> X-Mailer: Claws Mail 4.3.0 (GTK 3.24.43; x86_64-redhat-linux-gnu) 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 Content-Transfer-Encoding: 7bit On Thu, 22 May 2025 16:25:24 +0800 lizhe.67@bytedance.com wrote: > On Thu, 22 May 2025 09:22:50 +0200, david@redhat.com wrote: > > >On 22.05.25 05:49, lizhe.67@bytedance.com wrote: > >> On Wed, 21 May 2025 13:17:11 -0600, alex.williamson@redhat.com wrote: > >> > >>>> From: Li Zhe > >>>> > >>>> When vfio_pin_pages_remote() is called with a range of addresses that > >>>> includes large folios, the function currently performs individual > >>>> statistics counting operations for each page. This can lead to significant > >>>> performance overheads, especially when dealing with large ranges of pages. > >>>> > >>>> This patch optimize this process by batching the statistics counting > >>>> operations. > >>>> > >>>> The performance test results for completing the 8G VFIO IOMMU DMA mapping, > >>>> obtained through trace-cmd, are as follows. In this case, the 8G virtual > >>>> address space has been mapped to physical memory using hugetlbfs with > >>>> pagesize=2M. > >>>> > >>>> Before this patch: > >>>> funcgraph_entry: # 33813.703 us | vfio_pin_map_dma(); > >>>> > >>>> After this patch: > >>>> funcgraph_entry: # 16071.378 us | vfio_pin_map_dma(); > >>>> > >>>> Signed-off-by: Li Zhe > >>>> Co-developed-by: Alex Williamson > >>>> Signed-off-by: Alex Williamson > >>>> --- > >>> > >>> Given the discussion on v3, this is currently a Nak. Follow-up in that > >>> thread if there are further ideas how to salvage this. Thanks, > >> > >> How about considering the solution David mentioned to check whether the > >> pages or PFNs are actually consecutive? > >> > >> I have conducted a preliminary attempt, and the performance testing > >> revealed that the time consumption is approximately 18,000 microseconds. > >> Compared to the previous 33,000 microseconds, this also represents a > >> significant improvement. > >> > >> The modification is quite straightforward. The code below reflects the > >> changes I have made based on this patch. > >> > >> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c > >> index bd46ed9361fe..1cc1f76d4020 100644 > >> --- a/drivers/vfio/vfio_iommu_type1.c > >> +++ b/drivers/vfio/vfio_iommu_type1.c > >> @@ -627,6 +627,19 @@ static long vaddr_get_pfns(struct mm_struct *mm, unsigned long vaddr, > >> return ret; > >> } > >> > >> +static inline long continuous_page_num(struct vfio_batch *batch, long npage) > >> +{ > >> + long i; > >> + unsigned long next_pfn = page_to_pfn(batch->pages[batch->offset]) + 1; > >> + > >> + for (i = 1; i < npage; ++i) { > >> + if (page_to_pfn(batch->pages[batch->offset + i]) != next_pfn) > >> + break; > >> + next_pfn++; > >> + } > >> + return i; > >> +} > > > > > >What might be faster is obtaining the folio, and then calculating the > >next expected page pointer, comparing whether the page pointers match. > > > >Essentially, using folio_page() to calculate the expected next page. > > > >nth_page() is a simple pointer arithmetic with CONFIG_SPARSEMEM_VMEMMAP, > >so that might be rather fast. > > > > > >So we'd obtain > > > >start_idx = folio_idx(folio, batch->pages[batch->offset]); > > Do you mean using folio_page_idx()? > > >and then check for > > > >batch->pages[batch->offset + i] == folio_page(folio, start_idx + i) > > Thank you for your reminder. This is indeed a better solution. > The updated code might look like this: > > diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c > index bd46ed9361fe..f9a11b1d8433 100644 > --- a/drivers/vfio/vfio_iommu_type1.c > +++ b/drivers/vfio/vfio_iommu_type1.c > @@ -627,6 +627,20 @@ static long vaddr_get_pfns(struct mm_struct *mm, unsigned long vaddr, > return ret; > } > > +static inline long continuous_pages_num(struct folio *folio, > + struct vfio_batch *batch, long npage) Note this becomes long enough that we should just let the compiler decide whether to inline or not. > +{ > + long i; > + unsigned long start_idx = > + folio_page_idx(folio, batch->pages[batch->offset]); > + > + for (i = 1; i < npage; ++i) > + if (batch->pages[batch->offset + i] != > + folio_page(folio, start_idx + i)) > + break; > + return i; > +} > + > /* > * Attempt to pin pages. We really don't want to track all the pfns and > * the iommu can only map chunks of consecutive pfns anyway, so get the > @@ -708,8 +722,12 @@ static long vfio_pin_pages_remote(struct vfio_dma *dma, unsigned long vaddr, > */ > nr_pages = min_t(long, batch->size, folio_nr_pages(folio) - > folio_page_idx(folio, batch->pages[batch->offset])); > - if (nr_pages > 1 && vfio_find_vpfn_range(dma, iova, nr_pages)) > - nr_pages = 1; > + if (nr_pages > 1) { > + if (vfio_find_vpfn_range(dma, iova, nr_pages)) > + nr_pages = 1; > + else > + nr_pages = continuous_pages_num(folio, batch, nr_pages); > + } I think we can refactor this a bit better and maybe if we're going to the trouble of comparing pages we can be a bit more resilient to pages already accounted as vpfns. I took a shot at it, compile tested only, is there still a worthwhile gain? diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c index 0ac56072af9f..e8bba32148f7 100644 --- a/drivers/vfio/vfio_iommu_type1.c +++ b/drivers/vfio/vfio_iommu_type1.c @@ -319,7 +319,13 @@ static void vfio_dma_bitmap_free_all(struct vfio_iommu *iommu) /* * Helper Functions for host iova-pfn list */ -static struct vfio_pfn *vfio_find_vpfn(struct vfio_dma *dma, dma_addr_t iova) + +/* + * Find the first vfio_pfn that overlapping the range + * [iova_start, iova_end) in rb tree. + */ +static struct vfio_pfn *vfio_find_vpfn_range(struct vfio_dma *dma, + dma_addr_t iova_start, dma_addr_t iova_end) { struct vfio_pfn *vpfn; struct rb_node *node = dma->pfn_list.rb_node; @@ -327,9 +333,9 @@ static struct vfio_pfn *vfio_find_vpfn(struct vfio_dma *dma, dma_addr_t iova) while (node) { vpfn = rb_entry(node, struct vfio_pfn, node); - if (iova < vpfn->iova) + if (iova_end <= vpfn->iova) node = node->rb_left; - else if (iova > vpfn->iova) + else if (iova_start > vpfn->iova) node = node->rb_right; else return vpfn; @@ -337,6 +343,11 @@ static struct vfio_pfn *vfio_find_vpfn(struct vfio_dma *dma, dma_addr_t iova) return NULL; } +static inline struct vfio_pfn *vfio_find_vpfn(struct vfio_dma *dma, dma_addr_t iova) +{ + return vfio_find_vpfn_range(dma, iova, iova + PAGE_SIZE); +} + static void vfio_link_pfn(struct vfio_dma *dma, struct vfio_pfn *new) { @@ -615,6 +626,43 @@ static long vaddr_get_pfns(struct mm_struct *mm, unsigned long vaddr, return ret; } +static long contig_pages(struct vfio_dma *dma, + struct vfio_batch *batch, dma_addr_t iova) +{ + struct page *page = batch->pages[batch->offset]; + struct folio *folio = page_folio(page); + long idx = folio_page_idx(folio, page); + long max = min_t(long, batch->size, folio_nr_pages(folio) - idx); + long nr_pages; + + for (nr_pages = 1; nr_pages < max; nr_pages++) { + if (batch->pages[batch->offset + nr_pages] != + folio_page(folio, idx + nr_pages)) + break; + } + + return nr_pages; +} + +static long vpfn_pages(struct vfio_dma *dma, + dma_addr_t iova_start, long nr_pages) +{ + dma_addr_t iova_end = iova_start + (nr_pages << PAGE_SHIFT); + struct vfio_pfn *vpfn; + long count = 0; + + do { + vpfn = vfio_find_vpfn_range(dma, iova_start, iova_end); + if (likely(!vpfn)) + break; + + count++; + iova_start = vpfn->iova + PAGE_SIZE; + } while (iova_start < iova_end); + + return count; +} + /* * Attempt to pin pages. We really don't want to track all the pfns and * the iommu can only map chunks of consecutive pfns anyway, so get the @@ -681,32 +729,40 @@ static long vfio_pin_pages_remote(struct vfio_dma *dma, unsigned long vaddr, * and rsvd here, and therefore continues to use the batch. */ while (true) { + long nr_pages, acct_pages = 0; + if (pfn != *pfn_base + pinned || rsvd != is_invalid_reserved_pfn(pfn)) goto out; + nr_pages = contig_pages(dma, batch, iova); + if (!rsvd) { + acct_pages = nr_pages; + acct_pages -= vpfn_pages(dma, iova, nr_pages); + } + /* * Reserved pages aren't counted against the user, * externally pinned pages are already counted against * the user. */ - if (!rsvd && !vfio_find_vpfn(dma, iova)) { + if (acct_pages) { if (!dma->lock_cap && - mm->locked_vm + lock_acct + 1 > limit) { + mm->locked_vm + lock_acct + acct_pages > limit) { pr_warn("%s: RLIMIT_MEMLOCK (%ld) exceeded\n", __func__, limit << PAGE_SHIFT); ret = -ENOMEM; goto unpin_out; } - lock_acct++; + lock_acct += acct_pages; } - pinned++; - npage--; - vaddr += PAGE_SIZE; - iova += PAGE_SIZE; - batch->offset++; - batch->size--; + pinned += nr_pages; + npage -= nr_pages; + vaddr += PAGE_SIZE * nr_pages; + iova += PAGE_SIZE * nr_pages; + batch->offset += nr_pages; + batch->size -= nr_pages; if (!batch->size) break;