| Message ID | 20220620224936.52623-1-iii@linux.ibm.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | linux-user: Add partial support for MADV_DONTNEED | expand |
Le 21/06/2022 à 00:49, Ilya Leoshkevich a écrit : > Currently QEMU ignores madvise(MADV_DONTNEED), which break apps that > rely on this for zeroing out memory [1]. Improve the situation by doing > a passthrough when the range in question is a host-page-aligned > anonymous mapping. > > This is based on the patches from Simon Hausmann [2] and Chris Fallin > [3]. The structure is taken from Simon's patch. The PAGE_MAP_ANONYMOUS > bits are superseded by commit 26bab757d41b ("linux-user: Introduce > PAGE_ANON"). In the end the patch acts like the one from Chris: we > either pass-through the entire syscall, or do nothing, since doing this > only partially would not help the affected applications much. Finally, > add some extra checks to match the behavior of the Linux kernel [4]. > > [1] https://gitlab.com/qemu-project/qemu/-/issues/326 > [2] https://patchew.org/QEMU/20180827084037.25316-1-simon.hausmann@qt.io/ > [3] https://github.com/bytecodealliance/wasmtime/blob/v0.37.0/ci/qemu-madvise.patch > [4] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/mm/madvise.c?h=v5.19-rc3#n1368 > > Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com> > --- > linux-user/mmap.c | 68 ++++++++++++++++++++++++++++++++++++++++++ > linux-user/syscall.c | 6 +--- > linux-user/user-mmap.h | 1 + > 3 files changed, 70 insertions(+), 5 deletions(-) > > diff --git a/linux-user/mmap.c b/linux-user/mmap.c > index 48e1373796..900df7b28c 100644 > --- a/linux-user/mmap.c > +++ b/linux-user/mmap.c > @@ -835,3 +835,71 @@ abi_long target_mremap(abi_ulong old_addr, abi_ulong old_size, > mmap_unlock(); > return new_addr; > } > + > +static bool can_passthrough_madv_dontneed(abi_ulong start, abi_ulong end) > +{ > + ulong addr; > + > + if ((start | end) & ~qemu_host_page_mask) { > + return false; > + } > + > + for (addr = start; addr < end; addr += TARGET_PAGE_SIZE) { > + if (!(page_get_flags(addr) & PAGE_ANON)) { > + return false; > + } > + } > + > + return true; > +} > + > +int target_madvise(abi_ulong start, abi_ulong len_in, int advice) > +{ > + abi_ulong len, end; > + int ret = 0; > + > + if (start & ~TARGET_PAGE_MASK) { > + errno = EINVAL; > + return -1; You should remove the "errno = EINVAL" and return -TARGET_EINVAL. > + } > + len = TARGET_PAGE_ALIGN(len_in); > + > + if (len_in && !len) { > + errno = EINVAL; > + return -1; return -TARGET_EINVAL > + } > + > + end = start + len; > + if (end < start) { > + errno = EINVAL; > + return -1; return -TARGET_EINVAL > + } > + > + if (end == start) { > + return 0; > + } > + > + if (!guest_range_valid_untagged(start, len)) { > + errno = EINVAL; > + return -1; return -TARGET_EINVAL > + } > + > + /* > + * A straight passthrough may not be safe because qemu sometimes turns > + * private file-backed mappings into anonymous mappings. > + * > + * This is a hint, so ignoring and returning success is ok. > + * > + * This breaks MADV_DONTNEED, completely implementing which is quite > + * complicated. However, there is one low-hanging fruit: host-page-aligned > + * anonymous mappings. In this case passthrough is safe, so do it. > + */ > + mmap_lock(); > + if ((advice & MADV_DONTNEED) && > + can_passthrough_madv_dontneed(start, end)) { > + ret = madvise(g2h_untagged(start), len, MADV_DONTNEED); ret = get_errno(madvise(g2h_untagged(start), len, MADV_DONTNEED)); > + } > + mmap_unlock(); > + > + return ret; > +} > diff --git a/linux-user/syscall.c b/linux-user/syscall.c > index f55cdebee5..d25759b992 100644 > --- a/linux-user/syscall.c > +++ b/linux-user/syscall.c > @@ -11807,11 +11807,7 @@ static abi_long do_syscall1(CPUArchState *cpu_env, int num, abi_long arg1, > > #ifdef TARGET_NR_madvise > case TARGET_NR_madvise: > - /* A straight passthrough may not be safe because qemu sometimes > - turns private file-backed mappings into anonymous mappings. > - This will break MADV_DONTNEED. > - This is a hint, so ignoring and returning success is ok. */ > - return 0; > + return get_errno(target_madvise(arg1, arg2, arg3)); return target_madvise(arg1, arg2, arg3); > #endif > #ifdef TARGET_NR_fcntl64 > case TARGET_NR_fcntl64: > diff --git a/linux-user/user-mmap.h b/linux-user/user-mmap.h > index d1dec99c02..41cd358c7a 100644 > --- a/linux-user/user-mmap.h > +++ b/linux-user/user-mmap.h > @@ -25,6 +25,7 @@ int target_munmap(abi_ulong start, abi_ulong len); > abi_long target_mremap(abi_ulong old_addr, abi_ulong old_size, > abi_ulong new_size, unsigned long flags, > abi_ulong new_addr); > +int target_madvise(abi_ulong start, abi_ulong len_in, int advice); > extern unsigned long last_brk; > extern abi_ulong mmap_next_start; > abi_ulong mmap_find_vma(abi_ulong, abi_ulong, abi_ulong); Except comments above, it looks good. Reviewed-by: Laurent Vivier <laurent@vivier.eu>
diff --git a/linux-user/mmap.c b/linux-user/mmap.c index 48e1373796..900df7b28c 100644 --- a/linux-user/mmap.c +++ b/linux-user/mmap.c @@ -835,3 +835,71 @@ abi_long target_mremap(abi_ulong old_addr, abi_ulong old_size, mmap_unlock(); return new_addr; } + +static bool can_passthrough_madv_dontneed(abi_ulong start, abi_ulong end) +{ + ulong addr; + + if ((start | end) & ~qemu_host_page_mask) { + return false; + } + + for (addr = start; addr < end; addr += TARGET_PAGE_SIZE) { + if (!(page_get_flags(addr) & PAGE_ANON)) { + return false; + } + } + + return true; +} + +int target_madvise(abi_ulong start, abi_ulong len_in, int advice) +{ + abi_ulong len, end; + int ret = 0; + + if (start & ~TARGET_PAGE_MASK) { + errno = EINVAL; + return -1; + } + len = TARGET_PAGE_ALIGN(len_in); + + if (len_in && !len) { + errno = EINVAL; + return -1; + } + + end = start + len; + if (end < start) { + errno = EINVAL; + return -1; + } + + if (end == start) { + return 0; + } + + if (!guest_range_valid_untagged(start, len)) { + errno = EINVAL; + return -1; + } + + /* + * A straight passthrough may not be safe because qemu sometimes turns + * private file-backed mappings into anonymous mappings. + * + * This is a hint, so ignoring and returning success is ok. + * + * This breaks MADV_DONTNEED, completely implementing which is quite + * complicated. However, there is one low-hanging fruit: host-page-aligned + * anonymous mappings. In this case passthrough is safe, so do it. + */ + mmap_lock(); + if ((advice & MADV_DONTNEED) && + can_passthrough_madv_dontneed(start, end)) { + ret = madvise(g2h_untagged(start), len, MADV_DONTNEED); + } + mmap_unlock(); + + return ret; +} diff --git a/linux-user/syscall.c b/linux-user/syscall.c index f55cdebee5..d25759b992 100644 --- a/linux-user/syscall.c +++ b/linux-user/syscall.c @@ -11807,11 +11807,7 @@ static abi_long do_syscall1(CPUArchState *cpu_env, int num, abi_long arg1, #ifdef TARGET_NR_madvise case TARGET_NR_madvise: - /* A straight passthrough may not be safe because qemu sometimes - turns private file-backed mappings into anonymous mappings. - This will break MADV_DONTNEED. - This is a hint, so ignoring and returning success is ok. */ - return 0; + return get_errno(target_madvise(arg1, arg2, arg3)); #endif #ifdef TARGET_NR_fcntl64 case TARGET_NR_fcntl64: diff --git a/linux-user/user-mmap.h b/linux-user/user-mmap.h index d1dec99c02..41cd358c7a 100644 --- a/linux-user/user-mmap.h +++ b/linux-user/user-mmap.h @@ -25,6 +25,7 @@ int target_munmap(abi_ulong start, abi_ulong len); abi_long target_mremap(abi_ulong old_addr, abi_ulong old_size, abi_ulong new_size, unsigned long flags, abi_ulong new_addr); +int target_madvise(abi_ulong start, abi_ulong len_in, int advice); extern unsigned long last_brk; extern abi_ulong mmap_next_start; abi_ulong mmap_find_vma(abi_ulong, abi_ulong, abi_ulong);
Currently QEMU ignores madvise(MADV_DONTNEED), which break apps that rely on this for zeroing out memory [1]. Improve the situation by doing a passthrough when the range in question is a host-page-aligned anonymous mapping. This is based on the patches from Simon Hausmann [2] and Chris Fallin [3]. The structure is taken from Simon's patch. The PAGE_MAP_ANONYMOUS bits are superseded by commit 26bab757d41b ("linux-user: Introduce PAGE_ANON"). In the end the patch acts like the one from Chris: we either pass-through the entire syscall, or do nothing, since doing this only partially would not help the affected applications much. Finally, add some extra checks to match the behavior of the Linux kernel [4]. [1] https://gitlab.com/qemu-project/qemu/-/issues/326 [2] https://patchew.org/QEMU/20180827084037.25316-1-simon.hausmann@qt.io/ [3] https://github.com/bytecodealliance/wasmtime/blob/v0.37.0/ci/qemu-madvise.patch [4] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/mm/madvise.c?h=v5.19-rc3#n1368 Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com> --- linux-user/mmap.c | 68 ++++++++++++++++++++++++++++++++++++++++++ linux-user/syscall.c | 6 +--- linux-user/user-mmap.h | 1 + 3 files changed, 70 insertions(+), 5 deletions(-)