Message ID | 20210429214801.2583336-1-pcc@google.com (mailing list archive) |
---|---|
State | New, archived |
Headers | show |
Series | [v3] mm: improve mprotect(R|W) efficiency on pages referenced once | expand |
On Thu, Apr 29, 2021 at 02:48:01PM -0700, Peter Collingbourne wrote: > @@ -132,9 +134,12 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd, > } > > /* Avoid taking write faults for known dirty pages */ > - if (dirty_accountable && pte_dirty(ptent) && > - (pte_soft_dirty(ptent) || > - !(vma->vm_flags & VM_SOFTDIRTY))) { > + if ((dirty_accountable || > + (anon_writable && > + page_mapcount(pte_page(ptent)) == 1)) && > + pte_dirty(ptent) && > + (pte_soft_dirty(ptent) || > + !(vma->vm_flags & VM_SOFTDIRTY))) { > ptent = pte_mkwrite(ptent); > } > ptep_modify_prot_commit(vma, addr, pte, oldpte, ptent); This breaks uffd-wp anonymous page. I think we'd at least need below to be squashed: diff --git a/mm/mprotect.c b/mm/mprotect.c index ed10a790933e..6cb531eab811 100644 --- a/mm/mprotect.c +++ b/mm/mprotect.c @@ -139,6 +139,7 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd, (anon_writable && page_mapcount(pte_page(ptent)) == 1)) && pte_dirty(ptent) && + !pte_uffd_wp(ptent) && (pte_soft_dirty(ptent) || !(vma->vm_flags & VM_SOFTDIRTY))) { ptent = pte_mkwrite(ptent); It'll be good to start thinking about how to not continue growing this if clause condition, though..
diff --git a/mm/mprotect.c b/mm/mprotect.c index 94188df1ee55..3cf67de257e0 100644 --- a/mm/mprotect.c +++ b/mm/mprotect.c @@ -47,6 +47,8 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd, bool prot_numa = cp_flags & MM_CP_PROT_NUMA; bool uffd_wp = cp_flags & MM_CP_UFFD_WP; bool uffd_wp_resolve = cp_flags & MM_CP_UFFD_WP_RESOLVE; + bool anon_writable = + vma_is_anonymous(vma) && (vma->vm_flags & VM_WRITE); /* * Can be called with only the mmap_lock for reading by @@ -132,9 +134,12 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd, } /* Avoid taking write faults for known dirty pages */ - if (dirty_accountable && pte_dirty(ptent) && - (pte_soft_dirty(ptent) || - !(vma->vm_flags & VM_SOFTDIRTY))) { + if ((dirty_accountable || + (anon_writable && + page_mapcount(pte_page(ptent)) == 1)) && + pte_dirty(ptent) && + (pte_soft_dirty(ptent) || + !(vma->vm_flags & VM_SOFTDIRTY))) { ptent = pte_mkwrite(ptent); } ptep_modify_prot_commit(vma, addr, pte, oldpte, ptent);
In the Scudo memory allocator [1] we would like to be able to detect use-after-free vulnerabilities involving large allocations by issuing mprotect(PROT_NONE) on the memory region used for the allocation when it is deallocated. Later on, after the memory region has been "quarantined" for a sufficient period of time we would like to be able to use it for another allocation by issuing mprotect(PROT_READ|PROT_WRITE). Before this patch, after removing the write protection, any writes to the memory region would result in page faults and entering the copy-on-write code path, even in the usual case where the pages are only referenced by a single PTE, harming performance unnecessarily. Make it so that any pages in anonymous mappings that are only referenced by a single PTE are immediately made writable during the mprotect so that we can avoid the page faults. This program shows the critical syscall sequence that we intend to use in the allocator: #include <string.h> #include <sys/mman.h> enum { kSize = 131072 }; int main(int argc, char **argv) { char *addr = (char *)mmap(0, kSize, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0); for (int i = 0; i != 100000; ++i) { memset(addr, i, kSize); mprotect((void *)addr, kSize, PROT_NONE); mprotect((void *)addr, kSize, PROT_READ | PROT_WRITE); } } The effect of this patch on the above program was measured on a DragonBoard 845c by taking the median real time execution time of 10 runs. Before: 2.94s After: 0.66s The effect was also measured using one of the microbenchmarks that we normally use to benchmark the allocator [2], after modifying it to make the appropriate mprotect calls [3]. With an allocation size of 131072 bytes to trigger the allocator's "large allocation" code path the per-iteration time was measured as follows: Before: 27450ns After: 6010ns This patch means that we do more work during the mprotect call itself in exchange for less work when the pages are accessed. In the worst case, the pages are not accessed at all. The effect of this patch in such cases was measured using the following program: #include <string.h> #include <sys/mman.h> enum { kSize = 131072 }; int main(int argc, char **argv) { char *addr = (char *)mmap(0, kSize, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0); memset(addr, 1, kSize); for (int i = 0; i != 100000; ++i) { #ifdef PAGE_FAULT memset(addr + (i * 4096) % kSize, i, 4096); #endif mprotect((void *)addr, kSize, PROT_NONE); mprotect((void *)addr, kSize, PROT_READ | PROT_WRITE); } } With PAGE_FAULT undefined (0 pages touched after removing write protection) the median real time execution time of 100 runs was measured as follows: Before: 0.330260s After: 0.338836s With PAGE_FAULT defined (1 page touched) the measurements were as follows: Before: 0.438048s After: 0.355661s So it seems that even with a single page fault the new approach is faster. I saw similar results if I adjusted the programs to use a larger mapping size. With kSize = 1048576 I get these numbers with PAGE_FAULT undefined: Before: 1.428988s After: 1.512016s i.e. around 5.5%. And these with PAGE_FAULT defined: Before: 1.518559s After: 1.524417s i.e. about the same. What I think we may conclude from these results is that for smaller mappings the advantage of the previous approach, although measurable, is wiped out by a single page fault. I think we may expect that there should be at least one access resulting in a page fault (under the previous approach) after making the pages writable, since the program presumably made the pages writable for a reason. For larger mappings we may guesstimate that the new approach wins if the density of future page faults is > 0.4%. But for the mappings that are large enough for density to matter (not just the absolute number of page faults) it doesn't seem like the increase in mprotect latency would be very large relative to the total mprotect execution time. Signed-off-by: Peter Collingbourne <pcc@google.com> Link: https://linux-review.googlesource.com/id/I98d75ef90e20330c578871c87494d64b1df3f1b8 Link: [1] https://source.android.com/devices/tech/debug/scudo Link: [2] https://cs.android.com/android/platform/superproject/+/master:bionic/benchmarks/stdlib_benchmark.cpp;l=53;drc=e8693e78711e8f45ccd2b610e4dbe0b94d551cc9 Link: [3] https://github.com/pcc/llvm-project/commit/scudo-mprotect-secondary2 --- mm/mprotect.c | 11 ++++++++--- 1 file changed, 8 insertions(+), 3 deletions(-)