Re: [RFC] iomap: use huge zero folio in iomap_dio_zero

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On Fri, May 17, 2024 at 02:36:29PM +0200, Hannes Reinecke wrote:
> > +#define ZERO_FSB_SIZE (65536)
> > +#define ZERO_FSB_ORDER (get_order(ZERO_FSB_SIZE))
> > +extern struct page *zero_fs_block;
> > +
> >   /*
> >    * char_dev.c
> >    */
> But why?
> We already have a perfectly fine hugepage zero page in huge_memory.c.
> Shouldn't we rather export that one and use it?
> (Actually I have some patches for doing so...)

But we don't necessarily.  We only have it if
do_huge_pmd_anonymous_page() satisfies:

        if (!(vmf->flags & FAULT_FLAG_WRITE) &&
                        !mm_forbids_zeropage(vma->vm_mm) &&
                        transparent_hugepage_use_zero_page()) {

ie we've taken a page fault on a PMD hole in a VMA, that VMA permits
PMD mappings to exist, the page fault was for read, the
forbids-huge-zeropage isn't set for this vma, and using the hugetlb zero
page isn't forbidden.

I'd like to see stats for how much the PMD-zero-page is actually used,
because I suspect it's not really used very much.





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