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    SubjectRe: [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 <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/

    > + 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
    >

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