Re: [PATCH 01/18] mm: add a kunmap_local_dirty helper
On Fri, Jun 18, 2021 at 11:37:28AM +0800, Herbert Xu wrote: > On Thu, Jun 17, 2021 at 08:01:57PM -0700, Ira Weiny wrote: > > > > > + flush_kernel_dcache_page(__page); \ > > > > Is this required on 32bit systems? Why is kunmap_flush_on_unmap() not > > sufficient on 64bit systems? The normal kunmap_local() path does that. > > > > I'm sorry but I did not see a conclusion to my query on V1. Herbert implied > > the > > he just copied from the crypto code.[1] I'm concerned that this _dirty() > > call > > is just going to confuse the users of kmap even more. So why can't we get > > to > > the bottom of why flush_kernel_dcache_page() needs so much logic around it > > before complicating the general kernel users. > > > > I would like to see it go away if possible. > > This thread may be related: > > https://lwn.net/Articles/240249/ Interesting! Thanks! Digging around a bit more I found: https://lore.kernel.org/patchwork/patch/439637/ Auditing all the flush_dcache_page() arch code reveals that the mapping field is either unused, or is checked for NULL. Furthermore, all the implementations call page_mapping_file() which further limits the page to not be a swap page. All flush_kernel_dcache_page() implementations appears to operate the same way in all arch's which define that call. So I'm confident now that additional !PageSlab(__page) checks are not needed and this patch is unnecessary. Christoph, can we leave this out of the kmap API and just fold the flush_kernel_dcache_page() calls back into the bvec code? Unfortunately, I'm not convinced this can be handled completely by kunmap_local() nor the mem*_page() calls because there is a difference between flush_dcache_page() and flush_kernel_dcache_page() in most archs... [parisc being an exception which falls back to flush_kernel_dcache_page()]... It seems like the generic unmap path _should_ be able to determine which call to make based on the page but I'd have to look at that more. Ira
Re: [PATCH 01/18] mm: add a kunmap_local_dirty helper
On Thu, Jun 17, 2021 at 08:01:57PM -0700, Ira Weiny wrote: > > > + flush_kernel_dcache_page(__page); \ > > Is this required on 32bit systems? Why is kunmap_flush_on_unmap() not > sufficient on 64bit systems? The normal kunmap_local() path does that. > > I'm sorry but I did not see a conclusion to my query on V1. Herbert implied > the > he just copied from the crypto code.[1] I'm concerned that this _dirty() call > is just going to confuse the users of kmap even more. So why can't we get to > the bottom of why flush_kernel_dcache_page() needs so much logic around it > before complicating the general kernel users. > > I would like to see it go away if possible. This thread may be related: https://lwn.net/Articles/240249/ Cheers, -- Email: Herbert Xu Home Page: http://gondor.apana.org.au/~herbert/ PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
Re: [PATCH 01/18] mm: add a kunmap_local_dirty helper
On Tue, Jun 15, 2021 at 03:24:39PM +0200, Christoph Hellwig wrote:
> Add a helper that calls flush_kernel_dcache_page before unmapping the
> local mapping. flush_kernel_dcache_page is required for all pages
> potentially mapped into userspace that were written to using kmap*,
> so having a helper that does the right thing can be very convenient.
>
> Signed-off-by: Christoph Hellwig
> ---
> include/linux/highmem-internal.h | 7 +++
> include/linux/highmem.h | 4
> 2 files changed, 11 insertions(+)
>
> diff --git a/include/linux/highmem-internal.h
> b/include/linux/highmem-internal.h
> index 7902c7d8b55f..bd37706db147 100644
> --- a/include/linux/highmem-internal.h
> +++ b/include/linux/highmem-internal.h
> @@ -224,4 +224,11 @@ do {
> \
> __kunmap_local(__addr); \
> } while (0)
>
> +#define kunmap_local_dirty(__page, __addr) \
I think having to store the page and addr to return to kunmap_local_dirty() is
going to be a pain in some code paths. Not a show stopper but see below...
> +do { \
> + if (!PageSlab(__page)) \
Was there some clarification why the page can't be a Slab page? Or is this
just an optimization?
> + flush_kernel_dcache_page(__page); \
Is this required on 32bit systems? Why is kunmap_flush_on_unmap() not
sufficient on 64bit systems? The normal kunmap_local() path does that.
I'm sorry but I did not see a conclusion to my query on V1. Herbert implied the
he just copied from the crypto code.[1] I'm concerned that this _dirty() call
is just going to confuse the users of kmap even more. So why can't we get to
the bottom of why flush_kernel_dcache_page() needs so much logic around it
before complicating the general kernel users.
I would like to see it go away if possible.
Ira
[1] https://lore.kernel.org/lkml/20210615050258.ga5...@gondor.apana.org.au/
> + kunmap_local(__addr); \
> +} while (0)
> +
> #endif
> diff --git a/include/linux/highmem.h b/include/linux/highmem.h
> index 832b49b50c7b..65f548db4f2d 100644
> --- a/include/linux/highmem.h
> +++ b/include/linux/highmem.h
> @@ -93,6 +93,10 @@ static inline void kmap_flush_unused(void);
> * On HIGHMEM enabled systems mapping a highmem page has the side effect of
> * disabling migration in order to keep the virtual address stable across
> * preemption. No caller of kmap_local_page() can rely on this side effect.
> + *
> + * If data is written to the returned kernel mapping, the callers needs to
> + * unmap the mapping using kunmap_local_dirty(), else kunmap_local() should
> + * be used.
> */
> static inline void *kmap_local_page(struct page *page);
>
> --
> 2.30.2
>
[PATCH 01/18] mm: add a kunmap_local_dirty helper
Add a helper that calls flush_kernel_dcache_page before unmapping the
local mapping. flush_kernel_dcache_page is required for all pages
potentially mapped into userspace that were written to using kmap*,
so having a helper that does the right thing can be very convenient.
Signed-off-by: Christoph Hellwig
---
include/linux/highmem-internal.h | 7 +++
include/linux/highmem.h | 4
2 files changed, 11 insertions(+)
diff --git a/include/linux/highmem-internal.h b/include/linux/highmem-internal.h
index 7902c7d8b55f..bd37706db147 100644
--- a/include/linux/highmem-internal.h
+++ b/include/linux/highmem-internal.h
@@ -224,4 +224,11 @@ do {
\
__kunmap_local(__addr); \
} while (0)
+#define kunmap_local_dirty(__page, __addr) \
+do { \
+ if (!PageSlab(__page)) \
+ flush_kernel_dcache_page(__page); \
+ kunmap_local(__addr); \
+} while (0)
+
#endif
diff --git a/include/linux/highmem.h b/include/linux/highmem.h
index 832b49b50c7b..65f548db4f2d 100644
--- a/include/linux/highmem.h
+++ b/include/linux/highmem.h
@@ -93,6 +93,10 @@ static inline void kmap_flush_unused(void);
* On HIGHMEM enabled systems mapping a highmem page has the side effect of
* disabling migration in order to keep the virtual address stable across
* preemption. No caller of kmap_local_page() can rely on this side effect.
+ *
+ * If data is written to the returned kernel mapping, the callers needs to
+ * unmap the mapping using kunmap_local_dirty(), else kunmap_local() should
+ * be used.
*/
static inline void *kmap_local_page(struct page *page);
--
2.30.2
