Re: [PATCH 01/18] mm: add a kunmap_local_dirty helper
From: Ira Weiny <ira.weiny@intel.com>
Date: 2021-06-18 03:02:02
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ceph-devel, dm-devel, linux-arch, linux-block, linuxppc-dev, lkml
On Tue, Jun 15, 2021 at 03:24:39PM +0200, Christoph Hellwig wrote:
quoted hunk ↗ jump to hunk
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 <hch@lst.de> --- 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.GA5208@gondor.apana.org.au/ (local)
quoted hunk ↗ jump to hunk
+ kunmap_local(__addr); \ +} while (0) + #endifdiff --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