Re: [PATCH] Tight check of pfn_valid on sparsemem - v4

[Minchan Kim <minchan.kim@gmail.com>]

On Tue, Jul 27, 2010 at 5:12 PM, Milton Miller <miltonm@bga.com> wrote:
>
> On Tue Jul 27 2010 about 02:11:22 Minchan Kim wrote:
>> > [Sorry if i missed or added anyone on cc, patchwork.kernel.org  LKML is not
>> > working and I'm not subscribed to the list ]
>>
>> Readd them. :)
>
> Changed linux-mmc at vger to linxu-mm at kvack.org, from my poor use of grep
> MAINTAINERS.
>
>> On Tue, Jul 27, 2010 at 2:55 PM, <miltonm@xxxxxxx> wrote:
>> > On Mon Jul 26 2010 about 12:47:37 EST, Christoph Lameter wrote:
>> > > On Tue, 27 Jul 2010, Minchan Kim wrote:
>> > >
>> > > > This patch registers address of mem_section to memmap itself's page struct's
>> > > > pg->private field. This means the page is used for memmap of the section.
>> > > > Otherwise, the page is used for other purpose and memmap has a hole.
>> >
>> > >
>> > > > +void mark_valid_memmap(unsigned long start, unsigned long end);
>> > > > +
>> > > > +#ifdef CONFIG_ARCH_HAS_HOLES_MEMORYMODEL
>> > > > +static inline int memmap_valid(unsigned long pfn)
>> > > > +{
>> > > > + struct page *page = pfn_to_page(pfn);
>> > > > + struct page *__pg = virt_to_page(page);
>> > > > + return page_private(__pg) == (unsigned long)__pg;
>> > >
>> > >
>> > > What if page->private just happens to be the value of the page struct?
>> > > Even if that is not possible today, someday someone may add new
>> > > functionality to the kernel where page->pivage == page is used for some
>> > > reason.
>> > >
>> > > Checking for PG_reserved wont work?
>> >
>> > I had the same thought and suggest setting it to the memory section block,
>> > since that is a uniquie value (unlike PG_reserved),
>>
>> You mean setting pg->private to mem_section address?
>> I hope I understand your point.
>>
>> Actually, KAMEZAWA tried it at first version but I changed it.
>> That's because I want to support this mechanism to ARM FLATMEM.
>> (It doesn't have mem_section)
>
>> >
>> > .. and we already have computed it when we use it so we could pass it as
>> > a parameter (to both _valid and mark_valid).
>>
>> I hope this can support FALTMEM which have holes(ex, ARM).
>>
>
> If we pass a void * to this helper we should be able to find another
> symbol.  Looking at the pfn_valid() in arch/arm/mm/init.c I would
> probably choose &meminfo as it is already used nearby, and using a single

If we uses pg itself and PG_reserved, we can remove &meminfo in FLATMEM.

> symbol in would avoid issues if a more specific symbol chosen (eg bank)
> were to change at a pfn boundary not PAGE_SIZE / sizeof(struct page).
> Similarly the asm-generic/page.h version could use &max_mapnr.

I don't consider NOMMU.
I am not sure NOMMU have a this problem.

>
> This function is a validation helper for pfn_valid not the only check.
>
> something like
>
> static inline int memmap_valid(unsigned long pfn, void *validate)
> {
>        struct page *page = pfn_to_page(pfn);
>        struct page *__pg = virt_to_page(page);
>        return page_private(__pg) == validate;
> }

I am not sure what's benefit we have if we use validate argument.

>
> static inline int pfn_valid(unsigned long pfn)
> {
>        struct mem_section *ms;
>        if (pfn_to_section_nr(pfn) >= NR_MEM_SECTIONS)
>                return 0;
>        ms = __nr_to_section(pfn_to_section_nr(pfn));
>        return valid_section(ms) && memmap_valid(pfn, ms);
> }
>
>> > > > +/*
>> > > > + * Fill pg->private on valid mem_map with page itself.
>> > > > + * pfn_valid() will check this later. (see include/linux/mmzone.h)
>> > > > + * Every arch for supporting hole of mem_map should call
>> > > > + * mark_valid_memmap(start, end). please see usage in ARM.
>> > > > + */
>> > > > +void mark_valid_memmap(unsigned long start, unsigned long end)
>> > > > +{
>> > > > +       struct mem_section *ms;
>> > > > +       unsigned long pos, next;
>> > > > +       struct page *pg;
>> > > > +       void *memmap, *mapend;
>> > > > +
>> > > > +       for (pos = start; pos < end; pos = next) {
>> > > > +               next = (pos + PAGES_PER_SECTION) & PAGE_SECTION_MASK;
>> > > > +               ms = __pfn_to_section(pos);
>> > > > +               if (!valid_section(ms))
>> > > > +                       continue;
>> > > > +
>> > > > +               for (memmap = (void*)pfn_to_page(pos),
>> > > > +                                       /* The last page in section */
>> > > > +                                       mapend = pfn_to_page(next-1);
>> > > > +                               memmap < mapend; memmap += PAGE_SIZE) {
>> > > > +                       pg = virt_to_page(memmap);
>> > > > +                       set_page_private(pg, (unsigned long)pg);
>> > > > +               }
>> > > > +       }
>> > > > +}
>
> Hmm, this loop would need to change for sections.   And sizeof(struct
> page) % PAGE_SIZE may not be 0, so we want a global symbol for sparsemem

I can't understand your point. What is problem of sizeof(struct page)%PAGE_SIZE?
AFAIK, I believe sizeof(struct page) is always 32 bit in 32 bit
machine and most of PAGE_SIZE is 4K. What's problem happen?

> too.  Perhaps the mem_section array.  Using a symbol that is part of
> the model pre-checks can remove a global symbol lookup and has the side
> effect of making sure our pfn_valid is for the right model.

global symbol lookup?
Hmm, Please let me know your approach's benefit for improving this patch. :)

Thanks for careful review, milton.

-- 
Kind regards,
Minchan Kim

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