| Message ID | 20230125015738.912924-2-zokeefe@google.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | [1/2] mm/MADV_COLLAPSE: set EAGAIN on unexpected page refcount | expand |
Hi Zach,
Thank you for the patch! Yet something to improve:
[auto build test ERROR on akpm-mm/mm-everything]
url: https://github.com/intel-lab-lkp/linux/commits/Zach-O-Keefe/mm-MADV_COLLAPSE-catch-none-huge-bad-pmd-lookups/20230125-095954
base: https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-everything
patch link: https://lore.kernel.org/r/20230125015738.912924-2-zokeefe%40google.com
patch subject: [PATCH 2/2] mm/MADV_COLLAPSE: catch !none !huge !bad pmd lookups
config: x86_64-randconfig-r025-20230123 (https://download.01.org/0day-ci/archive/20230125/202301252033.HoFIRXm4-lkp@intel.com/config)
compiler: clang version 14.0.6 (https://github.com/llvm/llvm-project f28c006a5895fc0e329fe15fead81e37457cb1d1)
reproduce (this is a W=1 build):
wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
chmod +x ~/bin/make.cross
# https://github.com/intel-lab-lkp/linux/commit/6001eb9a8f1687a1d0b72831d991886106cac37b
git remote add linux-review https://github.com/intel-lab-lkp/linux
git fetch --no-tags linux-review Zach-O-Keefe/mm-MADV_COLLAPSE-catch-none-huge-bad-pmd-lookups/20230125-095954
git checkout 6001eb9a8f1687a1d0b72831d991886106cac37b
# save the config file
mkdir build_dir && cp config build_dir/.config
COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross W=1 O=build_dir ARCH=x86_64 olddefconfig
COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross W=1 O=build_dir ARCH=x86_64 SHELL=/bin/bash
If you fix the issue, kindly add following tag where applicable
| Reported-by: kernel test robot <lkp@intel.com>
All errors (new ones prefixed by >>):
>> mm/khugepaged.c:972:17: error: passing 'pmd_t **' to parameter of incompatible type 'pmd_t'
if (pmd_devmap(pmd))
^~~
arch/x86/include/asm/pgtable.h:254:36: note: passing argument to parameter 'pmd' here
static inline int pmd_devmap(pmd_t pmd)
^
1 error generated.
vim +972 mm/khugepaged.c
945
946 /*
947 * See pmd_trans_unstable() for how the result may change out from
948 * underneath us, even if we hold mmap_lock in read.
949 */
950 static int find_pmd_or_thp_or_none(struct mm_struct *mm,
951 unsigned long address,
952 pmd_t **pmd)
953 {
954 pmd_t pmde;
955
956 *pmd = mm_find_pmd(mm, address);
957 if (!*pmd)
958 return SCAN_PMD_NULL;
959
960 pmde = pmdp_get_lockless(*pmd);
961
962 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
963 /* See comments in pmd_none_or_trans_huge_or_clear_bad() */
964 barrier();
965 #endif
966 if (pmd_none(pmde))
967 return SCAN_PMD_NONE;
968 if (!pmd_present(pmde))
969 return SCAN_PMD_NULL;
970 if (pmd_trans_huge(pmde))
971 return SCAN_PMD_MAPPED;
> 972 if (pmd_devmap(pmd))
973 return SCAN_PMD_NULL;
974 if (pmd_bad(pmde))
975 return SCAN_PMD_NULL;
976 return SCAN_SUCCEED;
977 }
978
Hi Zach,
Thank you for the patch! Yet something to improve:
[auto build test ERROR on akpm-mm/mm-everything]
url: https://github.com/intel-lab-lkp/linux/commits/Zach-O-Keefe/mm-MADV_COLLAPSE-catch-none-huge-bad-pmd-lookups/20230125-095954
base: https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-everything
patch link: https://lore.kernel.org/r/20230125015738.912924-2-zokeefe%40google.com
patch subject: [PATCH 2/2] mm/MADV_COLLAPSE: catch !none !huge !bad pmd lookups
config: x86_64-rhel-8.3-kselftests (https://download.01.org/0day-ci/archive/20230125/202301252110.hFYRsbrm-lkp@intel.com/config)
compiler: gcc-11 (Debian 11.3.0-8) 11.3.0
reproduce (this is a W=1 build):
# https://github.com/intel-lab-lkp/linux/commit/6001eb9a8f1687a1d0b72831d991886106cac37b
git remote add linux-review https://github.com/intel-lab-lkp/linux
git fetch --no-tags linux-review Zach-O-Keefe/mm-MADV_COLLAPSE-catch-none-huge-bad-pmd-lookups/20230125-095954
git checkout 6001eb9a8f1687a1d0b72831d991886106cac37b
# save the config file
mkdir build_dir && cp config build_dir/.config
make W=1 O=build_dir ARCH=x86_64 olddefconfig
make W=1 O=build_dir ARCH=x86_64 SHELL=/bin/bash
If you fix the issue, kindly add following tag where applicable
| Reported-by: kernel test robot <lkp@intel.com>
All errors (new ones prefixed by >>):
mm/khugepaged.c: In function 'find_pmd_or_thp_or_none':
>> mm/khugepaged.c:972:24: error: incompatible type for argument 1 of 'pmd_devmap'
972 | if (pmd_devmap(pmd))
| ^~~
| |
| pmd_t **
In file included from include/linux/pgtable.h:6,
from include/linux/mm.h:29,
from mm/khugepaged.c:4:
arch/x86/include/asm/pgtable.h:254:36: note: expected 'pmd_t' but argument is of type 'pmd_t **'
254 | static inline int pmd_devmap(pmd_t pmd)
| ~~~~~~^~~
vim +/pmd_devmap +972 mm/khugepaged.c
945
946 /*
947 * See pmd_trans_unstable() for how the result may change out from
948 * underneath us, even if we hold mmap_lock in read.
949 */
950 static int find_pmd_or_thp_or_none(struct mm_struct *mm,
951 unsigned long address,
952 pmd_t **pmd)
953 {
954 pmd_t pmde;
955
956 *pmd = mm_find_pmd(mm, address);
957 if (!*pmd)
958 return SCAN_PMD_NULL;
959
960 pmde = pmdp_get_lockless(*pmd);
961
962 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
963 /* See comments in pmd_none_or_trans_huge_or_clear_bad() */
964 barrier();
965 #endif
966 if (pmd_none(pmde))
967 return SCAN_PMD_NONE;
968 if (!pmd_present(pmde))
969 return SCAN_PMD_NULL;
970 if (pmd_trans_huge(pmde))
971 return SCAN_PMD_MAPPED;
> 972 if (pmd_devmap(pmd))
973 return SCAN_PMD_NULL;
974 if (pmd_bad(pmde))
975 return SCAN_PMD_NULL;
976 return SCAN_SUCCEED;
977 }
978
diff --git a/mm/khugepaged.c b/mm/khugepaged.c index fa38cae240b9..7ea668bbea70 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -941,6 +941,10 @@ static int hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address, return SCAN_SUCCEED; } +/* + * See pmd_trans_unstable() for how the result may change out from + * underneath us, even if we hold mmap_lock in read. + */ static int find_pmd_or_thp_or_none(struct mm_struct *mm, unsigned long address, pmd_t **pmd) @@ -959,8 +963,12 @@ static int find_pmd_or_thp_or_none(struct mm_struct *mm, #endif if (pmd_none(pmde)) return SCAN_PMD_NONE; + if (!pmd_present(pmde)) + return SCAN_PMD_NULL; if (pmd_trans_huge(pmde)) return SCAN_PMD_MAPPED; + if (pmd_devmap(pmd)) + return SCAN_PMD_NULL; if (pmd_bad(pmde)) return SCAN_PMD_NULL; return SCAN_SUCCEED;
In commit 34488399fa08 ("mm/madvise: add file and shmem support to MADV_COLLAPSE") we make the following change to find_pmd_or_thp_or_none(): - if (!pmd_present(pmde)) - return SCAN_PMD_NULL; + if (pmd_none(pmde)) + return SCAN_PMD_NONE; This was for-use by MADV_COLLAPSE file/shmem codepaths, where MADV_COLLAPSE might identify a pte-mapped hugepage, only to have khugepaged race-in, free the pte table, and clear the pmd. Such codepaths include: A) If we find a suitably-aligned compound page of order HPAGE_PMD_ORDER already in the pagecache. B) In retract_page_tables(), if we fail to grab mmap_lock for the target mm/address. In these cases, collapse_pte_mapped_thp() really does expect a none (not just !present) pmd, and we want to suitably identify that case separate from the case where no pmd is found, or it's a bad-pmd (of course, many things could happen once we drop mmap_lock, and the pmd could plausibly undergo multiple transitions due to intervening fault, split, etc). Regardless, the code is prepared install a huge-pmd only when the existing pmd entry is either a genuine pte-table-mapping-pmd, or the none-pmd. However, the commit introduces a logical hole; namely, that we've allowed !none- && !huge- && !bad-pmds to be classified as genuine pte-table-mapping-pmds. One such example that could leak through are swap entries. The pmd values aren't checked again before use in pte_offset_map_lock(), which is expecting nothing less than a genuine pte-table-mapping-pmd. We want to put back the !pmd_present() check (below the pmd_none() check), but need to be careful to deal with subtleties in pmd transitions and treatments by various arch. The issue is that __split_huge_pmd_locked() temporarily clears the present bit (or otherwise marks the entry as invalid), but pmd_present() and pmd_trans_huge() still need to return true while the pmd is in this transitory state. For example, x86's pmd_present() also checks the _PAGE_PSE , riscv's version also checks the _PAGE_LEAF bit, and arm64 also checks a PMD_PRESENT_INVALID bit. Covering all 4 cases for x86 (all checks done on the same pmd value): 1) pmd_present() && pmd_trans_huge() All we actually know here is that the PSE bit is set. Either: a) We aren't racing with __split_huge_page(), and PRESENT or PROTNONE is set. => huge-pmd b) We are currently racing with __split_huge_page(). The danger here is that we proceed as-if we have a huge-pmd, but really we are looking at a pte-mapping-pmd. So, what is the risk of this danger? The only relevant path is: madvise_collapse() -> collapse_pte_mapped_thp() Where we might just incorrectly report back "success", when really the memory isn't pmd-backed. This is fine, since split could happen immediately after (actually) successful madvise_collapse(). So, it should be safe to just assume huge-pmd here. 2) pmd_present() && !pmd_trans_huge() Either: a) PSE not set and either PRESENT or PROTNONE is. => pte-table-mapping pmd (or PROT_NONE) b) devmap. This routine can be called immediately after unlocking/locking mmap_lock -- or called with no locks held (see khugepaged_scan_mm_slot()), so previous VMA checks have since been invalidated. 3) !pmd_present() && pmd_trans_huge() Not possible. 4) !pmd_present() && !pmd_trans_huge() Neither PRESENT nor PROTNONE set => not present I've checked all archs that implement pmd_trans_huge() (arm64, riscv, powerpc, longarch, x86, mips, s390) and this logic roughly translates (though devmap treatment is unique to x86 and powerpc, and (3) doesn't necessarily hold in general -- but that doesn't matter since !pmd_present() always takes failure path). Also, add a comment above find_pmd_or_thp_or_none() to help future travelers reason about the validity of the code; namely, the possible mutations that might happen out from under us, depending on how mmap_lock is held (if at all). Fixes: 34488399fa08 ("mm/madvise: add file and shmem support to MADV_COLLAPSE") Reported-by: Hugh Dickins <hughd@google.com> Signed-off-by: Zach O'Keefe <zokeefe@google.com> Cc: stable@vger.kernel.org --- Request that this be pulled into stable since it's theoretically possible (though I have no reproducer) that while mmap_lock is dropped, racing thp migration installs a pmd migration entry which then has a path to be consumed, unchecked, by pte_offset_map(). --- mm/khugepaged.c | 8 ++++++++ 1 file changed, 8 insertions(+)