linux-kernel - Re: [PATCH] export the function kmap_flush

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Date:	Fri, 08 Aug 2014 19:09:30 +0530
From:	Chintan Pandya <cpandya@...eaurora.org>
To:	"Sha, Ruibin" <ruibin.sha@...el.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"mel@....ul.ie" <mel@....ul.ie>,
	"a.p.zijlstra@...llo.nl" <a.p.zijlstra@...llo.nl>,
	"mgorman@...e.de" <mgorman@...e.de>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"Zhang, Yanmin" <yanmin.zhang@...el.com>,
	"He, Bo" <bo.he@...el.com>
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 ?

>
> 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@...el.com>
>
> ---
> 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 ?
>
> static inline unsigned long map_new_virtual(struct page *page)
> {
> --
> 1.7.9.5
>
> Best Regards
>
> ---------------------------------------------------------------
>
> Sha, Rui bin ( Robin )
>
> +86 13817890945
>
> Android System Integration Shanghai
>


-- 
Chintan Pandya

QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a
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