Messages in this thread | | | From | Nick Terrell <> | Subject | Re: [PATCH v2] lib/lz4: explicitly support in-place decompression | Date | Mon, 30 Nov 2020 18:19:34 +0000 |
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> On Nov 21, 2020, at 7:07 PM, Gao Xiang <hsiangkao@redhat.com> wrote: > > LZ4 final literal copy could be overlapped when doing > in-place decompression, so it's unsafe to just use memcpy() > on an optimized memcpy approach but memmove() instead. > > Upstream LZ4 has updated this years ago [1] (and the impact > is non-sensible [2] plus only a few bytes remain), this commit > just synchronizes LZ4 upstream code to the kernel side as well. > > It can be observed as EROFS in-place decompression failure > on specific files when X86_FEATURE_ERMS is unsupported, > memcpy() optimization of commit 59daa706fbec ("x86, mem: > Optimize memcpy by avoiding memory false dependece") will > be enabled then. > > Currently most modern x86-CPUs support ERMS, these CPUs just > use "rep movsb" approach so no problem at all. However, it can > still be verified with forcely disabling ERMS feature... > > arch/x86/lib/memcpy_64.S: > ALTERNATIVE_2 "jmp memcpy_orig", "", X86_FEATURE_REP_GOOD, \ > - "jmp memcpy_erms", X86_FEATURE_ERMS > + "jmp memcpy_orig", X86_FEATURE_ERMS > > We didn't observe any strange on arm64/arm/x86 platform before > since most memcpy() would behave in an increasing address order > ("copy upwards" [3]) and it's the correct order of in-place > decompression but it really needs an update to memmove() for sure > considering it's an undefined behavior according to the standard > and some unique optimization already exists in the kernel. > > [1] https://github.com/lz4/lz4/commit/33cb8518ac385835cc17be9a770b27b40cd0e15b > [2] https://github.com/lz4/lz4/pull/717#issuecomment-497818921 > [3] https://sourceware.org/bugzilla/show_bug.cgi?id=12518 > Cc: Yann Collet <yann.collet.73@gmail.com> > Cc: Nick Terrell <terrelln@fb.com> > Cc: Miao Xie <miaoxie@huawei.com> > Cc: Chao Yu <yuchao0@huawei.com> > Cc: Li Guifu <bluce.liguifu@huawei.com> > Cc: Guo Xuenan <guoxuenan@huawei.com> > Signed-off-by: Gao Xiang <hsiangkao@redhat.com> > --- > changes since v1: > - refine commit message; > - Cc more people. > > Hi Andrew, > > Could you kindly consider picking this patch up, although > the impact is EROFS but it touchs in-kernel lz4 library > anyway... > > Thanks, > Gao Xiang > > lib/lz4/lz4_decompress.c | 6 +++++- > lib/lz4/lz4defs.h | 1 + > 2 files changed, 6 insertions(+), 1 deletion(-) > > diff --git a/lib/lz4/lz4_decompress.c b/lib/lz4/lz4_decompress.c > index 00cb0d0b73e1..8a7724a6ce2f 100644 > --- a/lib/lz4/lz4_decompress.c > +++ b/lib/lz4/lz4_decompress.c > @@ -263,7 +263,11 @@ static FORCE_INLINE int LZ4_decompress_generic( > } > } > > - LZ4_memcpy(op, ip, length); > + /* > + * supports overlapping memory regions; only matters > + * for in-place decompression scenarios > + */ > + LZ4_memmove(op, ip, length); > ip += length; > op += length; > > diff --git a/lib/lz4/lz4defs.h b/lib/lz4/lz4defs.h > index c91dd96ef629..673bd206aa98 100644 > --- a/lib/lz4/lz4defs.h > +++ b/lib/lz4/lz4defs.h > @@ -146,6 +146,7 @@ static FORCE_INLINE void LZ4_writeLE16(void *memPtr, U16 value) > * environments. This is needed when decompressing the Linux Kernel, for example. > */ > #define LZ4_memcpy(dst, src, size) __builtin_memcpy(dst, src, size) > +#define LZ4_memmove(dst, src, size) __builtin_memmove(dst, src, size) > > static FORCE_INLINE void LZ4_copy8(void *dst, const void *src) > { > -- > 2.18.4 >
Looks good to me! You can add:
Reviewed-by: Nick Terrell <terrelln@fb.com>
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