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AJvYcCX96gaqkdPdmdRsSZiaY8H82wzKu7uBCShQ4RXU8sWjYeBKagVPAOrLUjelYv1p+HqOSbjsy1RpfA==@kvack.org X-Gm-Message-State: AOJu0YzrkFGjb+SUHe2E5YT71cHBZuLt9BBalX0wvAelFJ+LP4xigfz1 N5FqnyuMh+db1t4ynpYzjoWJxZrOOhItkdJvR8Lyu5o1mvKJkzvp+I3cT3HKa4WaagpOqpGiaE5 qrMRdlRyBMBJX9LfABMcuopaYPA== X-Google-Smtp-Source: AGHT+IFNpKZD9lN4cOQmrQeE/v6mJ+Aez5Zu8/h2NScD8jBwFV5IrxK/YzUb/E+mnOr3jMtt3fLdteDpb1Js1lKinQ== X-Received: from plhs4.prod.google.com ([2002:a17:903:3204:b0:223:5693:a4e9]) (user=ackerleytng job=prod-delivery.src-stubby-dispatcher) by 2002:a17:903:1a67:b0:234:d7b2:2ac2 with SMTP id d9443c01a7336-234d7b22c20mr46594455ad.22.1748450376728; Wed, 28 May 2025 09:39:36 -0700 (PDT) Date: Wed, 28 May 2025 09:39:35 -0700 In-Reply-To: Mime-Version: 1.0 References: <625bd9c98ad4fd49d7df678f0186129226f77d7d.1747264138.git.ackerleytng@google.com> Message-ID: Subject: Re: [RFC PATCH v2 39/51] KVM: guest_memfd: Merge and truncate on fallocate(PUNCH_HOLE) From: Ackerley Tng To: Yan Zhao Cc: kvm@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, x86@kernel.org, linux-fsdevel@vger.kernel.org, aik@amd.com, ajones@ventanamicro.com, akpm@linux-foundation.org, amoorthy@google.com, anthony.yznaga@oracle.com, anup@brainfault.org, aou@eecs.berkeley.edu, bfoster@redhat.com, binbin.wu@linux.intel.com, brauner@kernel.org, catalin.marinas@arm.com, chao.p.peng@intel.com, chenhuacai@kernel.org, dave.hansen@intel.com, david@redhat.com, dmatlack@google.com, dwmw@amazon.co.uk, erdemaktas@google.com, fan.du@intel.com, fvdl@google.com, graf@amazon.com, haibo1.xu@intel.com, hch@infradead.org, hughd@google.com, ira.weiny@intel.com, isaku.yamahata@intel.com, jack@suse.cz, james.morse@arm.com, jarkko@kernel.org, jgg@ziepe.ca, jgowans@amazon.com, jhubbard@nvidia.com, jroedel@suse.de, jthoughton@google.com, jun.miao@intel.com, kai.huang@intel.com, keirf@google.com, kent.overstreet@linux.dev, kirill.shutemov@intel.com, liam.merwick@oracle.com, maciej.wieczor-retman@intel.com, mail@maciej.szmigiero.name, maz@kernel.org, mic@digikod.net, michael.roth@amd.com, mpe@ellerman.id.au, muchun.song@linux.dev, nikunj@amd.com, nsaenz@amazon.es, oliver.upton@linux.dev, palmer@dabbelt.com, pankaj.gupta@amd.com, paul.walmsley@sifive.com, pbonzini@redhat.com, pdurrant@amazon.co.uk, peterx@redhat.com, pgonda@google.com, pvorel@suse.cz, qperret@google.com, quic_cvanscha@quicinc.com, quic_eberman@quicinc.com, quic_mnalajal@quicinc.com, quic_pderrin@quicinc.com, quic_pheragu@quicinc.com, quic_svaddagi@quicinc.com, quic_tsoni@quicinc.com, richard.weiyang@gmail.com, rick.p.edgecombe@intel.com, rientjes@google.com, roypat@amazon.co.uk, rppt@kernel.org, seanjc@google.com, shuah@kernel.org, steven.price@arm.com, steven.sistare@oracle.com, suzuki.poulose@arm.com, tabba@google.com, thomas.lendacky@amd.com, usama.arif@bytedance.com, vannapurve@google.com, vbabka@suse.cz, viro@zeniv.linux.org.uk, vkuznets@redhat.com, wei.w.wang@intel.com, will@kernel.org, willy@infradead.org, xiaoyao.li@intel.com, yilun.xu@intel.com, yuzenghui@huawei.com, zhiquan1.li@intel.com Content-Type: text/plain; charset="UTF-8" X-Rspamd-Queue-Id: 3E7D3180011 X-Stat-Signature: rwxhahcucorc5uyfqhd7zkfqjcgy93q9 X-Rspam-User: X-Rspamd-Server: rspam07 X-HE-Tag: 1748450377-989046 X-HE-Meta: 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 HwVCSYG+ 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 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: Yan Zhao writes: > On Wed, May 14, 2025 at 04:42:18PM -0700, Ackerley Tng wrote: >> Merge and truncate on fallocate(PUNCH_HOLE), but if the file is being >> closed, defer merging to folio_put() callback. >> >> Change-Id: Iae26987756e70c83f3b121edbc0ed0bc105eec0d >> Signed-off-by: Ackerley Tng >> --- >> virt/kvm/guest_memfd.c | 76 +++++++++++++++++++++++++++++++++++++----- >> 1 file changed, 68 insertions(+), 8 deletions(-) >> >> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c >> index cb426c1dfef8..04b1513c2998 100644 >> --- a/virt/kvm/guest_memfd.c >> +++ b/virt/kvm/guest_memfd.c >> @@ -859,6 +859,35 @@ static int kvm_gmem_restructure_folios_in_range(struct inode *inode, >> return ret; >> } >> >> +static long kvm_gmem_merge_truncate_indices(struct inode *inode, pgoff_t index, >> + size_t nr_pages) >> +{ >> + struct folio *f; >> + pgoff_t unused; >> + long num_freed; >> + >> + unmap_mapping_pages(inode->i_mapping, index, nr_pages, false); >> + >> + if (!kvm_gmem_has_safe_refcount(inode->i_mapping, index, nr_pages, &unused)) Yan, thank you for your reviews! > Why is kvm_gmem_has_safe_refcount() checked here, but not in > kvm_gmem_zero_range() within kvm_gmem_truncate_inode_range() in patch 33? > The contract for guest_memfd with HugeTLB pages is that if holes are punched in any ranges less than a full huge page, no pages are removed from the filemap. Those ranges are only zeroed. In kvm_gmem_zero_range(), we never remove any folios, and so there is no need to merge. If there's no need to merge, then we don't need to check for a safe refcount, and can just proceed to zero. kvm_gmem_merge_truncate_indices() is only used during hole punching and not when the file is closed. Hole punch vs file closure is checked using mapping_exiting(inode->i_mapping). During a hole punch, we will only allow truncation if there are no unexpected refcounts on any subpages, hence this kvm_gmem_has_safe_refcount() check. >> + return -EAGAIN; >> + > > Rather than merging the folios, could we simply call kvm_gmem_truncate_indices() > instead? > > num_freed = kvm_gmem_truncate_indices(inode->i_mapping, index, nr_pages); > return num_freed; > We could do this too, but then that would be deferring the huge page merging to the folio_put() callback and eventually the kernel worker thread. My goal here is to try to not to defer merging and freeing as much as possible so that most of the page/memory operations are synchronous, because synchronous operations are more predictable. As an example of improving predictability, in one of the selftests, I do a hole punch and then try to allocate again. Because the merging and freeing of the HugeTLB page sometimes takes too long, the allocation sometimes fails: the guest_memfd's subpool hadn't yet received the freed page back. With a synchronous truncation, the truncation may take longer, but the selftest predictably passes. >> + f = filemap_get_folio(inode->i_mapping, index); >> + if (IS_ERR(f)) >> + return 0; >> + >> + /* Leave just filemap's refcounts on the folio. */ >> + folio_put(f); >> + >> + WARN_ON(kvm_gmem_merge_folio_in_filemap(inode, f)); >> + >> + num_freed = folio_nr_pages(f); >> + folio_lock(f); >> + truncate_inode_folio(inode->i_mapping, f); >> + folio_unlock(f); >> + >> + return num_freed; >> +} >> + >> #else >> >> static inline int kvm_gmem_try_split_folio_in_filemap(struct inode *inode, >> @@ -874,6 +903,12 @@ static int kvm_gmem_restructure_folios_in_range(struct inode *inode, >> return 0; >> } >> >> +static long kvm_gmem_merge_truncate_indices(struct inode *inode, pgoff_t index, >> + size_t nr_pages) >> +{ >> + return 0; >> +} >> + >> #endif >> >> #else >> @@ -1182,8 +1217,10 @@ static long kvm_gmem_truncate_indices(struct address_space *mapping, >> * >> * Removes folios beginning @index for @nr_pages from filemap in @inode, updates >> * inode metadata. >> + * >> + * Return: 0 on success and negative error otherwise. >> */ >> -static void kvm_gmem_truncate_inode_aligned_pages(struct inode *inode, >> +static long kvm_gmem_truncate_inode_aligned_pages(struct inode *inode, >> pgoff_t index, >> size_t nr_pages) >> { >> @@ -1191,19 +1228,34 @@ static void kvm_gmem_truncate_inode_aligned_pages(struct inode *inode, >> long num_freed; >> pgoff_t idx; >> void *priv; >> + long ret; >> >> priv = kvm_gmem_allocator_private(inode); >> nr_per_huge_page = kvm_gmem_allocator_ops(inode)->nr_pages_in_folio(priv); >> >> + ret = 0; >> num_freed = 0; >> for (idx = index; idx < index + nr_pages; idx += nr_per_huge_page) { >> - num_freed += kvm_gmem_truncate_indices( >> - inode->i_mapping, idx, nr_per_huge_page); >> + if (mapping_exiting(inode->i_mapping) || >> + !kvm_gmem_has_some_shared(inode, idx, nr_per_huge_page)) { >> + num_freed += kvm_gmem_truncate_indices( >> + inode->i_mapping, idx, nr_per_huge_page); >> + } else { >> + ret = kvm_gmem_merge_truncate_indices(inode, idx, >> + nr_per_huge_page); >> + if (ret < 0) >> + break; >> + >> + num_freed += ret; >> + ret = 0; >> + } >> } >> >> spin_lock(&inode->i_lock); >> inode->i_blocks -= (num_freed << PAGE_SHIFT) / 512; >> spin_unlock(&inode->i_lock); >> + >> + return ret; >> } >> >> /** >> @@ -1252,8 +1304,10 @@ static void kvm_gmem_zero_range(struct address_space *mapping, >> * >> * Removes full (huge)pages from the filemap and zeroing incomplete >> * (huge)pages. The pages in the range may be split. >> + * >> + * Return: 0 on success and negative error otherwise. >> */ >> -static void kvm_gmem_truncate_inode_range(struct inode *inode, loff_t lstart, >> +static long kvm_gmem_truncate_inode_range(struct inode *inode, loff_t lstart, >> loff_t lend) >> { >> pgoff_t full_hpage_start; >> @@ -1263,6 +1317,7 @@ static void kvm_gmem_truncate_inode_range(struct inode *inode, loff_t lstart, >> pgoff_t start; >> pgoff_t end; >> void *priv; >> + long ret; >> >> priv = kvm_gmem_allocator_private(inode); >> nr_per_huge_page = kvm_gmem_allocator_ops(inode)->nr_pages_in_folio(priv); >> @@ -1279,10 +1334,11 @@ static void kvm_gmem_truncate_inode_range(struct inode *inode, loff_t lstart, >> kvm_gmem_zero_range(inode->i_mapping, start, zero_end); >> } >> >> + ret = 0; >> if (full_hpage_end > full_hpage_start) { >> nr_pages = full_hpage_end - full_hpage_start; >> - kvm_gmem_truncate_inode_aligned_pages(inode, full_hpage_start, >> - nr_pages); >> + ret = kvm_gmem_truncate_inode_aligned_pages( >> + inode, full_hpage_start, nr_pages); >> } >> >> if (end > full_hpage_end && end > full_hpage_start) { >> @@ -1290,6 +1346,8 @@ static void kvm_gmem_truncate_inode_range(struct inode *inode, loff_t lstart, >> >> kvm_gmem_zero_range(inode->i_mapping, zero_start, end); >> } >> + >> + return ret; >> } >> >> static long kvm_gmem_punch_hole(struct inode *inode, loff_t offset, loff_t len) >> @@ -1298,6 +1356,7 @@ static long kvm_gmem_punch_hole(struct inode *inode, loff_t offset, loff_t len) >> pgoff_t start = offset >> PAGE_SHIFT; >> pgoff_t end = (offset + len) >> PAGE_SHIFT; >> struct kvm_gmem *gmem; >> + long ret; >> >> /* >> * Bindings must be stable across invalidation to ensure the start+end >> @@ -1308,8 +1367,9 @@ static long kvm_gmem_punch_hole(struct inode *inode, loff_t offset, loff_t len) >> list_for_each_entry(gmem, gmem_list, entry) >> kvm_gmem_invalidate_begin_and_zap(gmem, start, end); >> >> + ret = 0; >> if (kvm_gmem_has_custom_allocator(inode)) { >> - kvm_gmem_truncate_inode_range(inode, offset, offset + len); >> + ret = kvm_gmem_truncate_inode_range(inode, offset, offset + len); >> } else { >> /* Page size is PAGE_SIZE, so use optimized truncation function. */ >> truncate_inode_pages_range(inode->i_mapping, offset, offset + len - 1); >> @@ -1320,7 +1380,7 @@ static long kvm_gmem_punch_hole(struct inode *inode, loff_t offset, loff_t len) >> >> filemap_invalidate_unlock(inode->i_mapping); >> >> - return 0; >> + return ret; >> } >> >> static long kvm_gmem_allocate(struct inode *inode, loff_t offset, loff_t len) >> -- >> 2.49.0.1045.g170613ef41-goog >>