Re: [PATCH v16 1/5] mm: add VM_DROPPABLE for designating always lazily freeable mappings

From: Jason A. Donenfeld
Date: Tue May 28 2024 - 16:52:02 EST

On Tue, May 28, 2024 at 01:41:50PM -0700, Frank van der Linden wrote:
> On Tue, May 28, 2024 at 5:24 AM Jason A. Donenfeld <Jason@xxxxxxxxx> wrote:
> >
> > 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@xxxxxxxxx
> > Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
> > ---
> > 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 | 8 +++++---
> > 8 files changed, 34 insertions(+), 4 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..d873a3f06506 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;
> > }
> > @@ -1851,7 +1852,8 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
> > * discarded. Remap the page to page table.
> > */
> > set_pte_at(mm, address, pvmw.pte, pteval);
> > - folio_set_swapbacked(folio);
> > + if (!(vma->vm_flags & VM_DROPPABLE))
> > + folio_set_swapbacked(folio);
> > ret = false;
> > page_vma_mapped_walk_done(&pvmw);
> > break;
> > --
> > 2.44.0
> >
> >
>
> This seems like an obvious question, but I can't seem to find a
> message asking this in the long history of this patchset: VM_DROPPABLE
> seems very close to MADV_FREE lazyfree memory.

Very different semantics and use case. For example, with MADV_FREE, if
you redirty the page by writing to it, the flag is cleared.

Jason