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(p200300d82f284600d3a76c2654cfe01e.dip0.t-ipconnect.de. [2003:d8:2f28:4600:d3a7:6c26:54cf:e01e]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3557dcf03acsm9229340f8f.103.2024.05.27.06.34.34 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 27 May 2024 06:34:34 -0700 (PDT) Message-ID: <2c943e16-e8f1-4311-b707-1be94d883948@redhat.com> Date: Mon, 27 May 2024 15:34:33 +0200 Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v15 1/5] mm: add VM_DROPPABLE for designating always lazily freeable mappings To: "Jason A. Donenfeld" , linux-kernel@vger.kernel.org, patches@lists.linux.dev, tglx@linutronix.de Cc: linux-mm@kvack.org References: <20240521111958.2384173-1-Jason@zx2c4.com> <20240521111958.2384173-2-Jason@zx2c4.com> From: David Hildenbrand In-Reply-To: <20240521111958.2384173-2-Jason@zx2c4.com> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Am 21.05.24 um 13:18 schrieb Jason A. Donenfeld: > The vDSO getrandom() implementation works with a buffer allocated with a > new system call that has certain requirements: > > - It shouldn't be written to core dumps. > * Easy: VM_DONTDUMP. > - It should be zeroed on fork. > * Easy: VM_WIPEONFORK. > > - It shouldn't be written to swap. > * Uh-oh: mlock is rlimited. > * Uh-oh: mlock isn't inherited by forks. > > - It shouldn't reserve actual memory, but it also shouldn't crash when > page faulting in memory if none is available > * Uh-oh: MAP_NORESERVE respects vm.overcommit_memory=2. > * Uh-oh: VM_NORESERVE means segfaults. > > It turns out that the vDSO getrandom() function has three really nice > characteristics that we can exploit to solve this problem: > > 1) Due to being wiped during fork(), the vDSO code is already robust to > having the contents of the pages it reads zeroed out midway through > the function's execution. > > 2) In the absolute worst case of whatever contingency we're coding for, > we have the option to fallback to the getrandom() syscall, and > everything is fine. > > 3) The buffers the function uses are only ever useful for a maximum of > 60 seconds -- a sort of cache, rather than a long term allocation. > > These characteristics mean that we can introduce VM_DROPPABLE, which > has the following semantics: > > a) It never is written out to swap. > b) Under memory pressure, mm can just drop the pages (so that they're > zero when read back again). > c) If there's not enough memory to service a page fault, it's not fatal. > d) It is inherited by fork. > e) It doesn't count against the mlock budget, since nothing is locked. > > This is fairly simple to implement, with the one snag that we have to > use 64-bit VM_* flags, but this shouldn't be a problem, since the only > consumers will probably be 64-bit anyway. > > This way, allocations used by vDSO getrandom() can use: > > VM_DROPPABLE | VM_DONTDUMP | VM_WIPEONFORK | VM_NORESERVE > > And there will be no problem with OOMing, crashing on overcommitment, > using memory when not in use, not wiping on fork(), coredumps, or > writing out to swap. > > Cc: linux-mm@kvack.org > Signed-off-by: Jason A. Donenfeld > --- > fs/proc/task_mmu.c | 3 +++ > include/linux/mm.h | 8 ++++++++ > include/trace/events/mmflags.h | 7 +++++++ > mm/Kconfig | 3 +++ > mm/memory.c | 4 ++++ > mm/mempolicy.c | 3 +++ > mm/mprotect.c | 2 +- > mm/rmap.c | 5 +++-- > 8 files changed, 32 insertions(+), 3 deletions(-) > > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c > index e5a5f015ff03..b5a59e57bde1 100644 > --- a/fs/proc/task_mmu.c > +++ b/fs/proc/task_mmu.c > @@ -706,6 +706,9 @@ static void show_smap_vma_flags(struct seq_file *m, struct vm_area_struct *vma) > #endif /* CONFIG_HAVE_ARCH_USERFAULTFD_MINOR */ > #ifdef CONFIG_X86_USER_SHADOW_STACK > [ilog2(VM_SHADOW_STACK)] = "ss", > +#endif > +#ifdef CONFIG_NEED_VM_DROPPABLE > + [ilog2(VM_DROPPABLE)] = "dp", > #endif > }; > size_t i; > diff --git a/include/linux/mm.h b/include/linux/mm.h > index 9849dfda44d4..5978cb4cc21c 100644 > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -321,12 +321,14 @@ extern unsigned int kobjsize(const void *objp); > #define VM_HIGH_ARCH_BIT_3 35 /* bit only usable on 64-bit architectures */ > #define VM_HIGH_ARCH_BIT_4 36 /* bit only usable on 64-bit architectures */ > #define VM_HIGH_ARCH_BIT_5 37 /* bit only usable on 64-bit architectures */ > +#define VM_HIGH_ARCH_BIT_6 38 /* bit only usable on 64-bit architectures */ > #define VM_HIGH_ARCH_0 BIT(VM_HIGH_ARCH_BIT_0) > #define VM_HIGH_ARCH_1 BIT(VM_HIGH_ARCH_BIT_1) > #define VM_HIGH_ARCH_2 BIT(VM_HIGH_ARCH_BIT_2) > #define VM_HIGH_ARCH_3 BIT(VM_HIGH_ARCH_BIT_3) > #define VM_HIGH_ARCH_4 BIT(VM_HIGH_ARCH_BIT_4) > #define VM_HIGH_ARCH_5 BIT(VM_HIGH_ARCH_BIT_5) > +#define VM_HIGH_ARCH_6 BIT(VM_HIGH_ARCH_BIT_6) > #endif /* CONFIG_ARCH_USES_HIGH_VMA_FLAGS */ > > #ifdef CONFIG_ARCH_HAS_PKEYS > @@ -357,6 +359,12 @@ extern unsigned int kobjsize(const void *objp); > # define VM_SHADOW_STACK VM_NONE > #endif > > +#ifdef CONFIG_NEED_VM_DROPPABLE > +# define VM_DROPPABLE VM_HIGH_ARCH_6 > +#else > +# define VM_DROPPABLE VM_NONE > +#endif > + > #if defined(CONFIG_X86) > # define VM_PAT VM_ARCH_1 /* PAT reserves whole VMA at once (x86) */ > #elif defined(CONFIG_PPC) > diff --git a/include/trace/events/mmflags.h b/include/trace/events/mmflags.h > index e46d6e82765e..fab7848df50a 100644 > --- a/include/trace/events/mmflags.h > +++ b/include/trace/events/mmflags.h > @@ -165,6 +165,12 @@ IF_HAVE_PG_ARCH_X(arch_3) > # define IF_HAVE_UFFD_MINOR(flag, name) > #endif > > +#ifdef CONFIG_NEED_VM_DROPPABLE > +# define IF_HAVE_VM_DROPPABLE(flag, name) {flag, name}, > +#else > +# define IF_HAVE_VM_DROPPABLE(flag, name) > +#endif > + > #define __def_vmaflag_names \ > {VM_READ, "read" }, \ > {VM_WRITE, "write" }, \ > @@ -197,6 +203,7 @@ IF_HAVE_VM_SOFTDIRTY(VM_SOFTDIRTY, "softdirty" ) \ > {VM_MIXEDMAP, "mixedmap" }, \ > {VM_HUGEPAGE, "hugepage" }, \ > {VM_NOHUGEPAGE, "nohugepage" }, \ > +IF_HAVE_VM_DROPPABLE(VM_DROPPABLE, "droppable" ) \ > {VM_MERGEABLE, "mergeable" } \ > > #define show_vma_flags(flags) \ > diff --git a/mm/Kconfig b/mm/Kconfig > index b4cb45255a54..6cd65ea4b3ad 100644 > --- a/mm/Kconfig > +++ b/mm/Kconfig > @@ -1056,6 +1056,9 @@ config ARCH_USES_HIGH_VMA_FLAGS > bool > config ARCH_HAS_PKEYS > bool > +config NEED_VM_DROPPABLE > + select ARCH_USES_HIGH_VMA_FLAGS > + bool > > config ARCH_USES_PG_ARCH_X > bool > diff --git a/mm/memory.c b/mm/memory.c > index b5453b86ec4b..57b03fc73159 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -5689,6 +5689,10 @@ vm_fault_t handle_mm_fault(struct vm_area_struct *vma, unsigned long address, > > lru_gen_exit_fault(); > > + /* If the mapping is droppable, then errors due to OOM aren't fatal. */ > + if (vma->vm_flags & VM_DROPPABLE) > + ret &= ~VM_FAULT_OOM; > + > if (flags & FAULT_FLAG_USER) { > mem_cgroup_exit_user_fault(); > /* > diff --git a/mm/mempolicy.c b/mm/mempolicy.c > index aec756ae5637..a66289f1d931 100644 > --- a/mm/mempolicy.c > +++ b/mm/mempolicy.c > @@ -2300,6 +2300,9 @@ struct folio *vma_alloc_folio_noprof(gfp_t gfp, int order, struct vm_area_struct > pgoff_t ilx; > struct page *page; > > + if (vma->vm_flags & VM_DROPPABLE) > + gfp |= __GFP_NOWARN | __GFP_NORETRY; > + > pol = get_vma_policy(vma, addr, order, &ilx); > page = alloc_pages_mpol_noprof(gfp | __GFP_COMP, order, > pol, ilx, numa_node_id()); > diff --git a/mm/mprotect.c b/mm/mprotect.c > index 94878c39ee32..88ff3ecc08a1 100644 > --- a/mm/mprotect.c > +++ b/mm/mprotect.c > @@ -622,7 +622,7 @@ mprotect_fixup(struct vma_iterator *vmi, struct mmu_gather *tlb, > may_expand_vm(mm, oldflags, nrpages)) > return -ENOMEM; > if (!(oldflags & (VM_ACCOUNT|VM_WRITE|VM_HUGETLB| > - VM_SHARED|VM_NORESERVE))) { > + VM_SHARED|VM_NORESERVE|VM_DROPPABLE))) { > charged = nrpages; > if (security_vm_enough_memory_mm(mm, charged)) > return -ENOMEM; > diff --git a/mm/rmap.c b/mm/rmap.c > index e8fc5ecb59b2..f23b9f796aae 100644 > --- a/mm/rmap.c > +++ b/mm/rmap.c > @@ -1397,7 +1397,8 @@ void folio_add_new_anon_rmap(struct folio *folio, struct vm_area_struct *vma, > VM_WARN_ON_FOLIO(folio_test_hugetlb(folio), folio); > VM_BUG_ON_VMA(address < vma->vm_start || > address + (nr << PAGE_SHIFT) > vma->vm_end, vma); > - __folio_set_swapbacked(folio); > + if (!(vma->vm_flags & VM_DROPPABLE)) > + __folio_set_swapbacked(folio); > __folio_set_anon(folio, vma, address, true); > > if (likely(!folio_test_large(folio))) { > @@ -1841,7 +1842,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma, > * plus the rmap(s) (dropped by discard:). > */ > if (ref_count == 1 + map_count && > - !folio_test_dirty(folio)) { > + (!folio_test_dirty(folio) || (vma->vm_flags & VM_DROPPABLE))) { > dec_mm_counter(mm, MM_ANONPAGES); > goto discard; > } If there is any unexpected temporary folio reference (ref_count != 1 + map_count), you'll end up calling folio_set_swapbacked() and the next swapout round will write that folio to swap. -- Thanks, David / dhildenb