Re: [PATCH] export the function kmap_flush_unused.

[Zhang, Yanmin]

On 2014/8/11 9:26, Sha, Ruibin wrote:
> Hi Chintan,
> Thank you very much for your timely and kindly response and comments.
>
> Here is more detail about our Scenario:
>
>      We have a big driver on Android product. The driver allocates lots of
>      DDR pages. When applications mmap a file exported from the driver,
>      driver would mmap the pages to the application space, usually with
>      uncachable prot.
>      On ia32/x86_64 arch, we have to avoid page cache alias issue. When
>      driver allocates the pages, it would change page original mapping in
>      page table with uncachable prot. Sometimes, the allocated page was
>      used by kmap/kunmap. After kunmap, the page is still mapped in KMAP
>      space. The entries in KMAP page table are not cleaned up until a
>      kernel thread flushes the freed KMAP pages(usually it is woken up by kunmap).
>      It means the driver need  force to flush the KMAP page table entries before mapping pages to
>      application space to be used. Otherwise, there is a race to create
>      cache alias.
>
>      To resolve this issue, we need export function kmap_flush_unused as
>      the driver is compiled as module. Then, the driver calls
>      kmap_flush_unused if the allocated pages are in HIGHMEM and being
>      used by kmap.
>
> Thanks again!
>
> Best Regards
> ---------------------------------------------------------------
> Sha, Rui bin ( Robin )
> +86 13817890945
> Android System Integration Shanghai
>
> -----Original Message-----
> From: Chintan Pandya [mailto:cpandya@xxxxxxxxxxxxxx]
> Sent: Friday, August 8, 2014 9:40 PM
> To: Sha, Ruibin
> Cc: linux-kernel@xxxxxxxxxxxxxxx; linux-mm@xxxxxxxxx; mel@xxxxxxxxx; a.p.zijlstra@xxxxxxxxx; mgorman@xxxxxxx; mingo@xxxxxxxxxx; Zhang, Yanmin; He, Bo
> Subject: Re: [PATCH] export the function kmap_flush_unused.
>
> On 08/08/2014 02:46 PM, Sha, Ruibin wrote:
> > export the function kmap_flush_unused.
> >
> > Scenario: When graphic driver need high memory spece, we use
> > alloc_pages() to allocate. But if the allocated page has just been
> > mapped in the KMAP space(like first kmap then kunmap) and no flush
> > page happened on PKMAP, the page virtual address is not NULL.Then when
> > we get that page and set page attribute like set_memory_uc and
> > set_memory_wc, we hit error.
> Could you explain your scenario with more details ? set_memory_* should be applied on mapped address. And in attempt to map your page (which was just kmap and kunmap'ed), it will overwrite the previous mappings.
>
> Moreover, in my view, kmap_flush_unused is just helping us in keeping the cache clean for kmap virtual addresses if they are unmapped. Is it serving any more purpose here ?

It depends on how to define 'clean' here. It resets pkmap_count[i] to 0,
and cleans up page table entries used by PKMAP. Here, our scenario is
caused by the late page table entry cleanup as driver need avoid page
cache alias.

> > fix: For that scenario,when we get the allocated page and its virtual
> > address is not NULL, we would like first flush that page.
> > So need export that function kmap_flush_unused.
> >
> > Signed-off-by: sha, ruibin <ruibin.sha@xxxxxxxxx>
> >
> > ---
> > mm/highmem.c | 1 +
> > 1 file changed, 1 insertion(+)
> >
> > diff --git a/mm/highmem.c b/mm/highmem.c
> > index b32b70c..511299b 100644
> > --- a/mm/highmem.c
> > +++ b/mm/highmem.c
> > @@ -156,6 +156,7 @@ void kmap_flush_unused(void)
> > flush_all_zero_pkmaps();
> > unlock_kmap();
> > }
> > +EXPORT_SYMBOL(kmap_flush_unused);
> This symbol is already extern'ed. Is it not sufficient for your case ?

We want to call it in driver module. extern is not enough.

Thanks,
Yanmin


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