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Donenfeld" To: David Hildenbrand Cc: linux-kernel@vger.kernel.org, patches@lists.linux.dev, tglx@linutronix.de, linux-mm@kvack.org Subject: Re: [PATCH v15 1/5] mm: add VM_DROPPABLE for designating always lazily freeable mappings Message-ID: References: <20240521111958.2384173-1-Jason@zx2c4.com> <20240521111958.2384173-2-Jason@zx2c4.com> <2c943e16-e8f1-4311-b707-1be94d883948@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <2c943e16-e8f1-4311-b707-1be94d883948@redhat.com> X-Stat-Signature: 6qsy56n4fxqs39mwxxrazgisuiqkeh1t X-Rspamd-Queue-Id: C883B120017 X-Rspam-User: X-Rspamd-Server: rspam10 X-HE-Tag: 1716897526-710830 X-HE-Meta: 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 PIG0oHJ1 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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: On Mon, May 27, 2024 at 03:34:33PM +0200, David Hildenbrand wrote: > > > 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, nice call. I'll guard the re-setting of folio_set_swapbacked() in that case. Jason