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linux/include/trace/events/mmflags.h
Jason A. Donenfeld 9651fcedf7 mm: add MAP_DROPPABLE for designating always lazily freeable mappings 
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: 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) It is inherited by fork.
d) It doesn't count against the mlock budget, since nothing is locked.
e) If there's not enough memory to service a page fault, it's not fatal,
   and no signal is sent.

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.

In order to let vDSO getrandom() use this, expose these via mmap(2) as
MAP_DROPPABLE.

Note that this involves removing the MADV_FREE special case from
sort_folio(), which according to Yu Zhao is unnecessary and will simply
result in an extra call to shrink_folio_list() in the worst case. The
chunk removed reenables the swapbacked flag, which we don't want for
VM_DROPPABLE, and we can't conditionalize it here because there isn't a
vma reference available.

Finally, the provided self test ensures that this is working as desired.

Cc: linux-mm@kvack.org
Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
2024-07-19 20:22:12 +02:00

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/* SPDX-License-Identifier: GPL-2.0 */
#include <linux/node.h>
#include <linux/mmzone.h>
#include <linux/compaction.h>
/*
* The order of these masks is important. Matching masks will be seen
* first and the left over flags will end up showing by themselves.
*
* For example, if we have GFP_KERNEL before GFP_USER we wil get:
*
* GFP_KERNEL|GFP_HARDWALL
*
* Thus most bits set go first.
*/
#define gfpflag_string(flag) {(__force unsigned long)flag, #flag}
#define __def_gfpflag_names \
gfpflag_string(GFP_TRANSHUGE), \
gfpflag_string(GFP_TRANSHUGE_LIGHT), \
gfpflag_string(GFP_HIGHUSER_MOVABLE), \
gfpflag_string(GFP_HIGHUSER), \
gfpflag_string(GFP_USER), \
gfpflag_string(GFP_KERNEL_ACCOUNT), \
gfpflag_string(GFP_KERNEL), \
gfpflag_string(GFP_NOFS), \
gfpflag_string(GFP_ATOMIC), \
gfpflag_string(GFP_NOIO), \
gfpflag_string(GFP_NOWAIT), \
gfpflag_string(GFP_DMA), \
gfpflag_string(__GFP_HIGHMEM), \
gfpflag_string(GFP_DMA32), \
gfpflag_string(__GFP_HIGH), \
gfpflag_string(__GFP_IO), \
gfpflag_string(__GFP_FS), \
gfpflag_string(__GFP_NOWARN), \
gfpflag_string(__GFP_RETRY_MAYFAIL), \
gfpflag_string(__GFP_NOFAIL), \
gfpflag_string(__GFP_NORETRY), \
gfpflag_string(__GFP_COMP), \
gfpflag_string(__GFP_ZERO), \
gfpflag_string(__GFP_NOMEMALLOC), \
gfpflag_string(__GFP_MEMALLOC), \
gfpflag_string(__GFP_HARDWALL), \
gfpflag_string(__GFP_THISNODE), \
gfpflag_string(__GFP_RECLAIMABLE), \
gfpflag_string(__GFP_MOVABLE), \
gfpflag_string(__GFP_ACCOUNT), \
gfpflag_string(__GFP_WRITE), \
gfpflag_string(__GFP_RECLAIM), \
gfpflag_string(__GFP_DIRECT_RECLAIM), \
gfpflag_string(__GFP_KSWAPD_RECLAIM), \
gfpflag_string(__GFP_ZEROTAGS)
#ifdef CONFIG_KASAN_HW_TAGS
#define __def_gfpflag_names_kasan , \
gfpflag_string(__GFP_SKIP_ZERO), \
gfpflag_string(__GFP_SKIP_KASAN)
#else
#define __def_gfpflag_names_kasan
#endif
#define show_gfp_flags(flags) \
(flags) ? __print_flags(flags, "|", \
__def_gfpflag_names __def_gfpflag_names_kasan \
) : "none"
#ifdef CONFIG_MMU
#define IF_HAVE_PG_MLOCK(_name) ,{1UL << PG_##_name, __stringify(_name)}
#else
#define IF_HAVE_PG_MLOCK(_name)
#endif
#ifdef CONFIG_ARCH_USES_PG_UNCACHED
#define IF_HAVE_PG_UNCACHED(_name) ,{1UL << PG_##_name, __stringify(_name)}
#else
#define IF_HAVE_PG_UNCACHED(_name)
#endif
#ifdef CONFIG_MEMORY_FAILURE
#define IF_HAVE_PG_HWPOISON(_name) ,{1UL << PG_##_name, __stringify(_name)}
#else
#define IF_HAVE_PG_HWPOISON(_name)
#endif
#if defined(CONFIG_PAGE_IDLE_FLAG) && defined(CONFIG_64BIT)
#define IF_HAVE_PG_IDLE(_name) ,{1UL << PG_##_name, __stringify(_name)}
#else
#define IF_HAVE_PG_IDLE(_name)
#endif
#ifdef CONFIG_ARCH_USES_PG_ARCH_X
#define IF_HAVE_PG_ARCH_X(_name) ,{1UL << PG_##_name, __stringify(_name)}
#else
#define IF_HAVE_PG_ARCH_X(_name)
#endif
#define DEF_PAGEFLAG_NAME(_name) { 1UL << PG_##_name, __stringify(_name) }
#define __def_pageflag_names \
DEF_PAGEFLAG_NAME(locked), \
DEF_PAGEFLAG_NAME(waiters), \
DEF_PAGEFLAG_NAME(error), \
DEF_PAGEFLAG_NAME(referenced), \
DEF_PAGEFLAG_NAME(uptodate), \
DEF_PAGEFLAG_NAME(dirty), \
DEF_PAGEFLAG_NAME(lru), \
DEF_PAGEFLAG_NAME(active), \
DEF_PAGEFLAG_NAME(workingset), \
DEF_PAGEFLAG_NAME(owner_priv_1), \
DEF_PAGEFLAG_NAME(arch_1), \
DEF_PAGEFLAG_NAME(reserved), \
DEF_PAGEFLAG_NAME(private), \
DEF_PAGEFLAG_NAME(private_2), \
DEF_PAGEFLAG_NAME(writeback), \
DEF_PAGEFLAG_NAME(head), \
DEF_PAGEFLAG_NAME(mappedtodisk), \
DEF_PAGEFLAG_NAME(reclaim), \
DEF_PAGEFLAG_NAME(swapbacked), \
DEF_PAGEFLAG_NAME(unevictable) \
IF_HAVE_PG_MLOCK(mlocked) \
IF_HAVE_PG_UNCACHED(uncached) \
IF_HAVE_PG_HWPOISON(hwpoison) \
IF_HAVE_PG_IDLE(idle) \
IF_HAVE_PG_IDLE(young) \
IF_HAVE_PG_ARCH_X(arch_2) \
IF_HAVE_PG_ARCH_X(arch_3)
#define show_page_flags(flags) \
(flags) ? __print_flags(flags, "|", \
__def_pageflag_names \
) : "none"
#define DEF_PAGETYPE_NAME(_name) { PG_##_name, __stringify(_name) }
#define __def_pagetype_names \
DEF_PAGETYPE_NAME(slab), \
DEF_PAGETYPE_NAME(hugetlb), \
DEF_PAGETYPE_NAME(offline), \
DEF_PAGETYPE_NAME(guard), \
DEF_PAGETYPE_NAME(table), \
DEF_PAGETYPE_NAME(buddy)
#if defined(CONFIG_X86)
#define __VM_ARCH_SPECIFIC_1 {VM_PAT, "pat" }
#elif defined(CONFIG_PPC)
#define __VM_ARCH_SPECIFIC_1 {VM_SAO, "sao" }
#elif defined(CONFIG_PARISC)
#define __VM_ARCH_SPECIFIC_1 {VM_GROWSUP, "growsup" }
#elif !defined(CONFIG_MMU)
#define __VM_ARCH_SPECIFIC_1 {VM_MAPPED_COPY,"mappedcopy" }
#else
#define __VM_ARCH_SPECIFIC_1 {VM_ARCH_1, "arch_1" }
#endif
#ifdef CONFIG_MEM_SOFT_DIRTY
#define IF_HAVE_VM_SOFTDIRTY(flag,name) {flag, name },
#else
#define IF_HAVE_VM_SOFTDIRTY(flag,name)
#endif
#ifdef CONFIG_HAVE_ARCH_USERFAULTFD_MINOR
# define IF_HAVE_UFFD_MINOR(flag, name) {flag, name},
#else
# define IF_HAVE_UFFD_MINOR(flag, name)
#endif
#ifdef CONFIG_64BIT
# 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" }, \
{VM_EXEC, "exec" }, \
{VM_SHARED, "shared" }, \
{VM_MAYREAD, "mayread" }, \
{VM_MAYWRITE, "maywrite" }, \
{VM_MAYEXEC, "mayexec" }, \
{VM_MAYSHARE, "mayshare" }, \
{VM_GROWSDOWN, "growsdown" }, \
{VM_UFFD_MISSING, "uffd_missing" }, \
IF_HAVE_UFFD_MINOR(VM_UFFD_MINOR, "uffd_minor" ) \
{VM_PFNMAP, "pfnmap" }, \
{VM_UFFD_WP, "uffd_wp" }, \
{VM_LOCKED, "locked" }, \
{VM_IO, "io" }, \
{VM_SEQ_READ, "seqread" }, \
{VM_RAND_READ, "randread" }, \
{VM_DONTCOPY, "dontcopy" }, \
{VM_DONTEXPAND, "dontexpand" }, \
{VM_LOCKONFAULT, "lockonfault" }, \
{VM_ACCOUNT, "account" }, \
{VM_NORESERVE, "noreserve" }, \
{VM_HUGETLB, "hugetlb" }, \
{VM_SYNC, "sync" }, \
__VM_ARCH_SPECIFIC_1 , \
{VM_WIPEONFORK, "wipeonfork" }, \
{VM_DONTDUMP, "dontdump" }, \
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) \
(flags) ? __print_flags(flags, "|", \
__def_vmaflag_names \
) : "none"
#ifdef CONFIG_COMPACTION
#define COMPACTION_STATUS \
EM( COMPACT_SKIPPED, "skipped") \
EM( COMPACT_DEFERRED, "deferred") \
EM( COMPACT_CONTINUE, "continue") \
EM( COMPACT_SUCCESS, "success") \
EM( COMPACT_PARTIAL_SKIPPED, "partial_skipped") \
EM( COMPACT_COMPLETE, "complete") \
EM( COMPACT_NO_SUITABLE_PAGE, "no_suitable_page") \
EM( COMPACT_NOT_SUITABLE_ZONE, "not_suitable_zone") \
EMe(COMPACT_CONTENDED, "contended")
/* High-level compaction status feedback */
#define COMPACTION_FAILED 1
#define COMPACTION_WITHDRAWN 2
#define COMPACTION_PROGRESS 3
#define compact_result_to_feedback(result) \
({ \
enum compact_result __result = result; \
(__result == COMPACT_COMPLETE) ? COMPACTION_FAILED : \
(__result == COMPACT_SUCCESS) ? COMPACTION_PROGRESS : COMPACTION_WITHDRAWN; \
})
#define COMPACTION_FEEDBACK \
EM(COMPACTION_FAILED, "failed") \
EM(COMPACTION_WITHDRAWN, "withdrawn") \
EMe(COMPACTION_PROGRESS, "progress")
#define COMPACTION_PRIORITY \
EM(COMPACT_PRIO_SYNC_FULL, "COMPACT_PRIO_SYNC_FULL") \
EM(COMPACT_PRIO_SYNC_LIGHT, "COMPACT_PRIO_SYNC_LIGHT") \
EMe(COMPACT_PRIO_ASYNC, "COMPACT_PRIO_ASYNC")
#else
#define COMPACTION_STATUS
#define COMPACTION_PRIORITY
#define COMPACTION_FEEDBACK
#endif
#ifdef CONFIG_ZONE_DMA
#define IFDEF_ZONE_DMA(X) X
#else
#define IFDEF_ZONE_DMA(X)
#endif
#ifdef CONFIG_ZONE_DMA32
#define IFDEF_ZONE_DMA32(X) X
#else
#define IFDEF_ZONE_DMA32(X)
#endif
#ifdef CONFIG_HIGHMEM
#define IFDEF_ZONE_HIGHMEM(X) X
#else
#define IFDEF_ZONE_HIGHMEM(X)
#endif
#define ZONE_TYPE \
IFDEF_ZONE_DMA( EM (ZONE_DMA, "DMA")) \
IFDEF_ZONE_DMA32( EM (ZONE_DMA32, "DMA32")) \
EM (ZONE_NORMAL, "Normal") \
IFDEF_ZONE_HIGHMEM( EM (ZONE_HIGHMEM,"HighMem")) \
EMe(ZONE_MOVABLE,"Movable")
#define LRU_NAMES \
EM (LRU_INACTIVE_ANON, "inactive_anon") \
EM (LRU_ACTIVE_ANON, "active_anon") \
EM (LRU_INACTIVE_FILE, "inactive_file") \
EM (LRU_ACTIVE_FILE, "active_file") \
EMe(LRU_UNEVICTABLE, "unevictable")
/*
* First define the enums in the above macros to be exported to userspace
* via TRACE_DEFINE_ENUM().
*/
#undef EM
#undef EMe
#define EM(a, b) TRACE_DEFINE_ENUM(a);
#define EMe(a, b) TRACE_DEFINE_ENUM(a);
COMPACTION_STATUS
COMPACTION_PRIORITY
/* COMPACTION_FEEDBACK are defines not enums. Not needed here. */
ZONE_TYPE
LRU_NAMES
/*
* Now redefine the EM() and EMe() macros to map the enums to the strings
* that will be printed in the output.
*/
#undef EM
#undef EMe
#define EM(a, b) {a, b},
#define EMe(a, b) {a, b}
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