[PATCH] iomap: Add missing flush_dcache_page

[PATCH] iomap: Add missing flush_dcache_page

[Matthew Wilcox (Oracle)]

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(+)

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);
 }
 
-- 
2.30.2

Re: [PATCH] iomap: Add missing flush_dcache_page

[Christoph Hellwig]

On Fri, Jul 16, 2021 at 04:00:32PM +0100, Matthew Wilcox (Oracle) wrote:
> 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(+)
> 
> 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);

.. 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.

Re: [PATCH] iomap: Add missing flush_dcache_page

[Matthew Wilcox]

On Fri, Jul 16, 2021 at 04:04:18PM +0100, Christoph Hellwig wrote:
> On Fri, Jul 16, 2021 at 04:00:32PM +0100, Matthew Wilcox (Oracle) wrote:
> > 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(+)
> > 
> > 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);
> 
> .. 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.

Re: [PATCH] iomap: Add missing flush_dcache_page

[Christoph Hellwig]

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).

Re: [PATCH] iomap: Add missing flush_dcache_page

[Matthew Wilcox]

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).

Re: [PATCH] iomap: Add missing flush_dcache_page

[Gao Xiang]

On Fri, Jul 16, 2021 at 04:00:32PM +0100, Matthew Wilcox (Oracle) wrote:
> 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>

Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
(will update on my side as well)

Thanks,
Gao Xiang

> ---
>  fs/iomap/buffered-io.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> 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);
>  }
>  
> -- 
> 2.30.2