| Message ID | 20210716150032.1089982-1-willy@infradead.org |
|---|---|
| State | New |
| Headers | show |
| Series | iomap: Add missing flush_dcache_page | expand |
On Fri, Jul 16, 2021 at 06:28:10PM +0100, Matthew Wilcox wrote: > > > memcpy(addr, iomap->inline_data, size); > > > memset(addr + size, 0, PAGE_SIZE - size); > > > kunmap_atomic(addr); > > > + flush_dcache_page(page); > > > > .. and all writes into a kmap also need such a flush, so this needs to > > move a line up. My plan was to add a memcpy_to_page_and_pad helper > > ala memcpy_to_page to get various file systems and drivers out of the > > business of cache flushing as much as we can. > > hm? It's absolutely allowed to flush the page after calling kunmap. > Look at zero_user_segments(), for example. Documentation/core-api/cachetlb.rst states that any user page obtained using kmap needs a flush_kernel_dcache_page after modification. flush_dcache_page is a strict superset of flush_kernel_dcache_page. That beeing said flushing after kmap updates is a complete mess. arm as probably the poster child for dcache challenged plus highmem architectures always flushed caches from kunmap and, and arc has a flush_dcache_page that doesn't work at all on a highmem page that is not kmapped (where kmap_atomic and kmap_local_page don't count as kmapped as they don't set page->virtual).
On Mon, Jul 19, 2021 at 09:39:17AM +0100, Christoph Hellwig wrote: > On Fri, Jul 16, 2021 at 06:28:10PM +0100, Matthew Wilcox wrote: > > > > memcpy(addr, iomap->inline_data, size); > > > > memset(addr + size, 0, PAGE_SIZE - size); > > > > kunmap_atomic(addr); > > > > + flush_dcache_page(page); > > > > > > .. and all writes into a kmap also need such a flush, so this needs to > > > move a line up. My plan was to add a memcpy_to_page_and_pad helper > > > ala memcpy_to_page to get various file systems and drivers out of the > > > business of cache flushing as much as we can. > > > > hm? It's absolutely allowed to flush the page after calling kunmap. > > Look at zero_user_segments(), for example. > > Documentation/core-api/cachetlb.rst states that any user page obtained > using kmap needs a flush_kernel_dcache_page after modification. > flush_dcache_page is a strict superset of flush_kernel_dcache_page. Looks like (the other) Christoph broke this in 2008 with commit eebd2aa35569 ("Pagecache zeroing: zero_user_segment, zero_user_segments and zero_user"): It has one line about it in the changelog: Also extract the flushing of the caches to be outside of the kmap. ... which doesn't even attempt to justify why it's safe to do so. - memset((char *)kaddr + (offset), 0, (size)); \ - flush_dcache_page(page); \ - kunmap_atomic(kaddr, (km_type)); \ + kunmap_atomic(kaddr, KM_USER0); + flush_dcache_page(page); Looks like it came from https://lore.kernel.org/lkml/20070911060425.472862098@sgi.com/ but there was no discussion of this ... plenty of discussion about other conceptual problems with the entire patchset. > That beeing said flushing after kmap updates is a complete mess. > arm as probably the poster child for dcache challenged plus highmem > architectures always flushed caches from kunmap and, and arc has > a flush_dcache_page that doesn't work at all on a highmem page that > is not kmapped (where kmap_atomic and kmap_local_page don't count as > kmapped as they don't set page->virtual).
diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c index 41da4f14c00b..fe60c603f4ca 100644 --- a/fs/iomap/buffered-io.c +++ b/fs/iomap/buffered-io.c @@ -222,6 +222,7 @@ iomap_read_inline_data(struct inode *inode, struct page *page, memcpy(addr, iomap->inline_data, size); memset(addr + size, 0, PAGE_SIZE - size); kunmap_atomic(addr); + flush_dcache_page(page); SetPageUptodate(page); }
Inline data needs to be flushed from the kernel's view of a page before it's mapped by userspace. Cc: stable@vger.kernel.org Fixes: 19e0c58f6552 ("iomap: generic inline data handling") Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org> --- fs/iomap/buffered-io.c | 1 + 1 file changed, 1 insertion(+)