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SubjectRe: [PATCH v2] page cache: Store only head pages in i_pages
On Thu, Feb 14, 2019 at 12:53:31PM -0800, Matthew Wilcox wrote:
> On Thu, Feb 14, 2019 at 04:30:04PM +0300, Kirill A. Shutemov wrote:
> > - page_cache_delete_batch() will blow up on
> >
> > VM_BUG_ON_PAGE(page->index + HPAGE_PMD_NR - tail_pages
> > != pvec->pages[i]->index, page);
>
> Quite right. I decided to rewrite page_cache_delete_batch. What do you
> (and Jan!) think to this? Compile-tested only.
>
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 0d71b1acf811..facaa6913ffa 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -279,11 +279,11 @@ EXPORT_SYMBOL(delete_from_page_cache);
> * @pvec: pagevec with pages to delete
> *
> * The function walks over mapping->i_pages and removes pages passed in @pvec
> - * from the mapping. The function expects @pvec to be sorted by page index.
> + * from the mapping. The function expects @pvec to be sorted by page index
> + * and is optimised for it to be dense.
> * It tolerates holes in @pvec (mapping entries at those indices are not
> * modified). The function expects only THP head pages to be present in the
> - * @pvec and takes care to delete all corresponding tail pages from the
> - * mapping as well.
> + * @pvec.
> *
> * The function expects the i_pages lock to be held.
> */
> @@ -292,40 +292,36 @@ static void page_cache_delete_batch(struct address_space *mapping,
> {
> XA_STATE(xas, &mapping->i_pages, pvec->pages[0]->index);
> int total_pages = 0;
> - int i = 0, tail_pages = 0;
> + int i = 0;
> struct page *page;
>
> mapping_set_update(&xas, mapping);
> xas_for_each(&xas, page, ULONG_MAX) {
> - if (i >= pagevec_count(pvec) && !tail_pages)
> + if (i >= pagevec_count(pvec))
> break;
> +
> + /* A swap/dax/shadow entry got inserted? Skip it. */
> if (xa_is_value(page))
> continue;
> - if (!tail_pages) {
> - /*
> - * Some page got inserted in our range? Skip it. We
> - * have our pages locked so they are protected from
> - * being removed.
> - */
> - if (page != pvec->pages[i]) {
> - VM_BUG_ON_PAGE(page->index >
> - pvec->pages[i]->index, page);
> - continue;
> - }
> - WARN_ON_ONCE(!PageLocked(page));
> - if (PageTransHuge(page) && !PageHuge(page))
> - tail_pages = HPAGE_PMD_NR - 1;
> + /*
> + * A page got inserted in our range? Skip it. We have our
> + * pages locked so they are protected from being removed.
> + */
> + if (page != pvec->pages[i]) {

Maybe a comment for the VM_BUG while you're there?

> + VM_BUG_ON_PAGE(page->index > pvec->pages[i]->index,
> + page);
> + continue;
> + }
> +
> + WARN_ON_ONCE(!PageLocked(page));
> +
> + if (page->index == xas.xa_index)
> page->mapping = NULL;
> - /*
> - * Leave page->index set: truncation lookup relies
> - * upon it
> - */
> + /* Leave page->index set: truncation lookup relies on it */
> +
> + if (page->index + (1UL << compound_order(page)) - 1 ==
> + xas.xa_index)

It's 1am here and I'm slow, but it took me few minutes to understand how
it works. Please add a comment.

> i++;
> - } else {
> - VM_BUG_ON_PAGE(page->index + HPAGE_PMD_NR - tail_pages
> - != pvec->pages[i]->index, page);
> - tail_pages--;
> - }
> xas_store(&xas, NULL);
> total_pages++;
> }

--
Kirill A. Shutemov

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