Flush dcache before writing into page to avoid alias
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From: anfei on 25 Jan 2010 08:40 Hi Andrew, On Thu, Jan 21, 2010 at 01:07:57PM +0800, anfei zhou wrote: > The cache alias problem will happen if the changes of user shared mapping > is not flushed before copying, then user and kernel mapping may be mapped > into two different cache line, it is impossible to guarantee the coherence > after iov_iter_copy_from_user_atomic. So the right steps should be: > flush_dcache_page(page); > kmap_atomic(page); > write to page; > kunmap_atomic(page); > flush_dcache_page(page); > More precisely, we might create two new APIs flush_dcache_user_page and > flush_dcache_kern_page to replace the two flush_dcache_page accordingly. > > Here is a snippet tested on omap2430 with VIPT cache, and I think it is > not ARM-specific: > int val = 0x11111111; > fd = open("abc", O_RDWR); > addr = mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0); > *(addr+0) = 0x44444444; > tmp = *(addr+0); > *(addr+1) = 0x77777777; > write(fd, &val, sizeof(int)); > close(fd); > The results are not always 0x11111111 0x77777777 at the beginning as expected. > Is this a real bug or not necessary to support? Thanks, Anfei. > Signed-off-by: Anfei <anfei.zhou(a)gmail.com> > --- > fs/fuse/file.c | 3 +++ > mm/filemap.c | 3 +++ > 2 files changed, 6 insertions(+), 0 deletions(-) > > diff --git a/fs/fuse/file.c b/fs/fuse/file.c > index c18913a..a9f5e13 100644 > --- a/fs/fuse/file.c > +++ b/fs/fuse/file.c > @@ -828,6 +828,9 @@ static ssize_t fuse_fill_write_pages(struct fuse_req *req, > if (!page) > break; > > + if (mapping_writably_mapped(mapping)) > + flush_dcache_page(page); > + > pagefault_disable(); > tmp = iov_iter_copy_from_user_atomic(page, ii, offset, bytes); > pagefault_enable(); > diff --git a/mm/filemap.c b/mm/filemap.c > index 96ac6b0..07056fb 100644 > --- a/mm/filemap.c > +++ b/mm/filemap.c > @@ -2196,6 +2196,9 @@ again: > if (unlikely(status)) > break; > > + if (mapping_writably_mapped(mapping)) > + flush_dcache_page(page); > + > pagefault_disable(); > copied = iov_iter_copy_from_user_atomic(page, i, offset, bytes); > pagefault_enable(); > -- > 1.6.3.1 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Andrew Morton on 25 Jan 2010 15:00 On Mon, 25 Jan 2010 21:33:08 +0800 anfei <anfei.zhou(a)gmail.com> wrote: > Hi Andrew, > > On Thu, Jan 21, 2010 at 01:07:57PM +0800, anfei zhou wrote: > > The cache alias problem will happen if the changes of user shared mapping > > is not flushed before copying, then user and kernel mapping may be mapped > > into two different cache line, it is impossible to guarantee the coherence > > after iov_iter_copy_from_user_atomic. So the right steps should be: > > flush_dcache_page(page); > > kmap_atomic(page); > > write to page; > > kunmap_atomic(page); > > flush_dcache_page(page); > > More precisely, we might create two new APIs flush_dcache_user_page and > > flush_dcache_kern_page to replace the two flush_dcache_page accordingly. > > > > Here is a snippet tested on omap2430 with VIPT cache, and I think it is > > not ARM-specific: > > int val = 0x11111111; > > fd = open("abc", O_RDWR); > > addr = mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0); > > *(addr+0) = 0x44444444; > > tmp = *(addr+0); > > *(addr+1) = 0x77777777; > > write(fd, &val, sizeof(int)); > > close(fd); > > The results are not always 0x11111111 0x77777777 at the beginning as expected. > > > Is this a real bug or not necessary to support? Bug. If variable `addr' has type int* then the contents of that file should be 0x11111111 0x77777777. You didn't tell us what the contents were in the incorrect case, but I guess it doesn't matter. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Russell King - ARM Linux on 25 Jan 2010 15:10 On Mon, Jan 25, 2010 at 11:58:14AM -0800, Andrew Morton wrote: > On Mon, 25 Jan 2010 21:33:08 +0800 anfei <anfei.zhou(a)gmail.com> wrote: > > > Hi Andrew, > > > > On Thu, Jan 21, 2010 at 01:07:57PM +0800, anfei zhou wrote: > > > The cache alias problem will happen if the changes of user shared mapping > > > is not flushed before copying, then user and kernel mapping may be mapped > > > into two different cache line, it is impossible to guarantee the coherence > > > after iov_iter_copy_from_user_atomic. So the right steps should be: > > > flush_dcache_page(page); > > > kmap_atomic(page); > > > write to page; > > > kunmap_atomic(page); > > > flush_dcache_page(page); > > > More precisely, we might create two new APIs flush_dcache_user_page and > > > flush_dcache_kern_page to replace the two flush_dcache_page accordingly. > > > > > > Here is a snippet tested on omap2430 with VIPT cache, and I think it is > > > not ARM-specific: > > > int val = 0x11111111; > > > fd = open("abc", O_RDWR); > > > addr = mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0); > > > *(addr+0) = 0x44444444; > > > tmp = *(addr+0); > > > *(addr+1) = 0x77777777; > > > write(fd, &val, sizeof(int)); > > > close(fd); > > > The results are not always 0x11111111 0x77777777 at the beginning as expected. > > > > > Is this a real bug or not necessary to support? > > Bug. If variable `addr' has type int* then the contents of that file > should be 0x11111111 0x77777777. You didn't tell us what the contents > were in the incorrect case, but I guess it doesn't matter. FYI, from a previous email from anfei: 0x44444444 0x77777777 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: anfei zhou on 25 Jan 2010 20:10 On Tue, Jan 26, 2010 at 3:58 AM, Andrew Morton <akpm(a)linux-foundation.org> wrote: > On Mon, 25 Jan 2010 21:33:08 +0800 anfei <anfei.zhou(a)gmail.com> wrote: > >> Hi Andrew, >> >> On Thu, Jan 21, 2010 at 01:07:57PM +0800, anfei zhou wrote: >> > The cache alias problem will happen if the changes of user shared mapping >> > is not flushed before copying, then user and kernel mapping may be mapped >> > into two different cache line, it is impossible to guarantee the coherence >> > after iov_iter_copy_from_user_atomic. �So the right steps should be: >> > � � flush_dcache_page(page); >> > � � kmap_atomic(page); >> > � � write to page; >> > � � kunmap_atomic(page); >> > � � flush_dcache_page(page); >> > More precisely, we might create two new APIs flush_dcache_user_page and >> > flush_dcache_kern_page to replace the two flush_dcache_page accordingly. >> > >> > Here is a snippet tested on omap2430 with VIPT cache, and I think it is >> > not ARM-specific: >> > � � int val = 0x11111111; >> > � � fd = open("abc", O_RDWR); >> > � � addr = mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0); >> > � � *(addr+0) = 0x44444444; >> > � � tmp = *(addr+0); >> > � � *(addr+1) = 0x77777777; >> > � � write(fd, &val, sizeof(int)); >> > � � close(fd); >> > The results are not always 0x11111111 0x77777777 at the beginning as expected. >> > >> Is this a real bug or not necessary to support? > > Bug. �If variable `addr' has type int* then the contents of that file > should be 0x11111111 0x77777777. �You didn't tell us what the contents > were in the incorrect case, but I guess it doesn't matter. > Sorry, I didn't give the details, here is the old thread with more details: http://linux.derkeiler.com/Mailing-Lists/Kernel/2010-01/msg07124.html Regards, Anfei. > > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Sebastian Andrzej Siewior on 27 Jan 2010 17:10
* Russell King - ARM Linux | 2010-01-25 20:00:04 [+0000]: >On Mon, Jan 25, 2010 at 11:58:14AM -0800, Andrew Morton wrote: >> On Mon, 25 Jan 2010 21:33:08 +0800 anfei <anfei.zhou(a)gmail.com> wrote: >> >> > Hi Andrew, >> > >> > On Thu, Jan 21, 2010 at 01:07:57PM +0800, anfei zhou wrote: >> > > The cache alias problem will happen if the changes of user shared mapping >> > > is not flushed before copying, then user and kernel mapping may be mapped >> > > into two different cache line, it is impossible to guarantee the coherence >> > > after iov_iter_copy_from_user_atomic. So the right steps should be: >> > > flush_dcache_page(page); >> > > kmap_atomic(page); >> > > write to page; >> > > kunmap_atomic(page); >> > > flush_dcache_page(page); >> > > More precisely, we might create two new APIs flush_dcache_user_page and >> > > flush_dcache_kern_page to replace the two flush_dcache_page accordingly. >> > > >> > > Here is a snippet tested on omap2430 with VIPT cache, and I think it is >> > > not ARM-specific: >> > > int val = 0x11111111; >> > > fd = open("abc", O_RDWR); >> > > addr = mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0); >> > > *(addr+0) = 0x44444444; >> > > tmp = *(addr+0); >> > > *(addr+1) = 0x77777777; >> > > write(fd, &val, sizeof(int)); >> > > close(fd); >> > > The results are not always 0x11111111 0x77777777 at the beginning as expected. >> > > >> > Is this a real bug or not necessary to support? >> >> Bug. If variable `addr' has type int* then the contents of that file >> should be 0x11111111 0x77777777. You didn't tell us what the contents >> were in the incorrect case, but I guess it doesn't matter. > >FYI, from a previous email from anfei: > >0x44444444 0x77777777 I just wanted to query what the status of this patch is. This patch seems to fix a real bug which causes a test suite to fail on ARM [0]. The test suite passes on my VIVT ARM with this patch. [0] http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=524003 Sebastian -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ |