Re: [PATCH v2] mm/highmem: make kmap cache coloring aware

[Max Filippov <jcmvbkbc@gmail.com>]

Hi David,

On Tue, Jul 22, 2014 at 4:58 AM, David Rientjes <rientjes@google.com> wrote:
> On Thu, 17 Jul 2014, Max Filippov wrote:
>
>> From: Leonid Yegoshin <Leonid.Yegoshin@imgtec.com>
>>
>> Provide hooks that allow architectures with aliasing cache to align
>> mapping address of high pages according to their color. Such architectures
>> may enforce similar coloring of low- and high-memory page mappings and
>> reuse existing cache management functions to support highmem.
>>
>
> Typically a change like this would be proposed along with a change to an
> architecture which would define this new ARCH_PKMAP_COLORING and have its
> own overriding definitions.  Based on who you sent this patch to, it looks
> like that would be mips and xtensa.  Now the only question is where are
> those patches to add the alternate definitions for those platforms?

I'm going to post xtensa series shortly, and I saw corresponding changes
for MIPS in the following patch:
http://permalink.gmane.org/gmane.linux.kernel.mm/107654

>> Signed-off-by: Leonid Yegoshin <Leonid.Yegoshin@imgtec.com>
>> [ Max: extract architecture-independent part of the original patch, clean
>>   up checkpatch and build warnings. ]
>> Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
>> ---
>> Changes v1->v2:
>> - fix description
>>
>>  mm/highmem.c | 19 ++++++++++++++++---
>>  1 file changed, 16 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/highmem.c b/mm/highmem.c
>> index b32b70c..6898a8b 100644
>> --- a/mm/highmem.c
>> +++ b/mm/highmem.c
>> @@ -44,6 +44,14 @@ DEFINE_PER_CPU(int, __kmap_atomic_idx);
>>   */
>>  #ifdef CONFIG_HIGHMEM
>>
>> +#ifndef ARCH_PKMAP_COLORING
>> +#define set_pkmap_color(pg, cl)              /* */
>
> This is typically done with do {} while (0).

Will fix.

>> +#define get_last_pkmap_nr(p, cl)     (p)
>> +#define get_next_pkmap_nr(p, cl)     (((p) + 1) & LAST_PKMAP_MASK)
>> +#define is_no_more_pkmaps(p, cl)     (!(p))
>
> That's not gramatically proper.

Just no_more_pkmaps maybe?

>> +#define get_next_pkmap_counter(c, cl)        ((c) - 1)
>> +#endif
>> +
>>  unsigned long totalhigh_pages __read_mostly;
>>  EXPORT_SYMBOL(totalhigh_pages);
>>
>> @@ -161,19 +169,24 @@ static inline unsigned long map_new_virtual(struct page *page)
>>  {
>>       unsigned long vaddr;
>>       int count;
>> +     int color __maybe_unused;
>> +
>> +     set_pkmap_color(page, color);
>> +     last_pkmap_nr = get_last_pkmap_nr(last_pkmap_nr, color);
>>
>>  start:
>>       count = LAST_PKMAP;
>>       /* Find an empty entry */
>>       for (;;) {
>> -             last_pkmap_nr = (last_pkmap_nr + 1) & LAST_PKMAP_MASK;
>> -             if (!last_pkmap_nr) {
>> +             last_pkmap_nr = get_next_pkmap_nr(last_pkmap_nr, color);
>> +             if (is_no_more_pkmaps(last_pkmap_nr, color)) {
>>                       flush_all_zero_pkmaps();
>>                       count = LAST_PKMAP;
>>               }
>>               if (!pkmap_count[last_pkmap_nr])
>>                       break;  /* Found a usable entry */
>> -             if (--count)
>> +             count = get_next_pkmap_counter(count, color);
>
> And that's not equivalent at all, --count decrements the auto variable and
> then tests it for being non-zero.  Your get_next_pkmap_counter() never
> decrements count.

Yes, it's not literally equivalent, but it does the same thing: count gets
decreased by 1 and for loop iterates until count reaches zero.
get_next_pkmap_counter has no side effects, which is good. And count
is not supposed to go below zero anyway, but sure I can change the below
condition to 'if (count)'.

>> +             if (count > 0)
>>                       continue;
>>
>>               /*

-- 
Thanks.
-- Max

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