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From: Matthew Wilcox <willy@infradead.org>
To: Christoph Hellwig <hch@infradead.org>
Cc: linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 02/25] mm: Optimise find_subpage for !THP
Date: Wed, 12 Feb 2020 05:02:00 -0800
Message-ID: <20200212130200.GC7778@bombadil.infradead.org> (raw)
In-Reply-To: <20200212074105.GE7068@infradead.org>

On Tue, Feb 11, 2020 at 11:41:05PM -0800, Christoph Hellwig wrote:
> On Tue, Feb 11, 2020 at 08:18:22PM -0800, Matthew Wilcox wrote:
> > From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> > 
> > If THP is disabled, find_subpage can become a no-op.
> > 
> > Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> > ---
> >  include/linux/pagemap.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> > index 75bdfec49710..0842622cca90 100644
> > --- a/include/linux/pagemap.h
> > +++ b/include/linux/pagemap.h
> > @@ -340,7 +340,7 @@ static inline struct page *find_subpage(struct page *page, pgoff_t offset)
> >  
> >  	VM_BUG_ON_PAGE(PageTail(page), page);
> >  
> > -	return page + (offset & (compound_nr(page) - 1));
> > +	return page + (offset & (hpage_nr_pages(page) - 1));
> >  }
> 
> So just above the quoted code is a
> 
> 	if (PageHuge(page))
> 		return page;
> 
> So how can we get into a compound page that is not a huge page, but
> only if THP is enabled?  (Yes, maybe I'm a little rusty on VM
> internals).

That's for hugetlbfs:

        if (!PageCompound(page))
                return 0;

        page = compound_head(page);
        return page[1].compound_dtor == HUGETLB_PAGE_DTOR;

> Can you add comments describing the use case of this function and why
> it does all these checks?  It looks like black magic to me.

Would this help?

-static inline struct page *find_subpage(struct page *page, pgoff_t offset)
+/*
+ * Given the page we found in the page cache, return the page corresponding
+ * to this offset in the file
+ */
+static inline struct page *find_subpage(struct page *head, pgoff_t offset)
 {
-       if (PageHuge(page))
-               return page;
+       /* HugeTLBfs wants the head page regardless */
+       if (PageHuge(head))
+               return head;
 
-       VM_BUG_ON_PAGE(PageTail(page), page);
+       VM_BUG_ON_PAGE(PageTail(head), head);
 
-       return page + (offset & (hpage_nr_pages(page) - 1));
+       return head + (offset & (hpage_nr_pages(head) - 1));
 }


  reply index

Thread overview: 68+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-12  4:18 [PATCH v2 00/25] Large pages in the page cache Matthew Wilcox
2020-02-12  4:18 ` [PATCH v2 01/25] mm: Use vm_fault error code directly Matthew Wilcox
2020-02-12  7:34   ` Christoph Hellwig
2020-02-12  4:18 ` [PATCH v2 02/25] mm: Optimise find_subpage for !THP Matthew Wilcox
2020-02-12  7:41   ` Christoph Hellwig
2020-02-12 13:02     ` Matthew Wilcox [this message]
2020-02-12 17:52       ` Christoph Hellwig
2020-02-13 13:50       ` Kirill A. Shutemov
2020-02-12  4:18 ` [PATCH v2 03/25] mm: Use VM_BUG_ON_PAGE in clear_page_dirty_for_io Matthew Wilcox
2020-02-12  7:38   ` Christoph Hellwig
2020-02-13 13:50   ` Kirill A. Shutemov
2020-02-12  4:18 ` [PATCH v2 04/25] mm: Unexport find_get_entry Matthew Wilcox
2020-02-12  7:37   ` Christoph Hellwig
2020-02-13 13:51   ` Kirill A. Shutemov
2020-02-12  4:18 ` [PATCH v2 05/25] mm: Fix documentation of FGP flags Matthew Wilcox
2020-02-12  7:42   ` Christoph Hellwig
2020-02-12 19:11     ` Matthew Wilcox
2020-02-13 14:00       ` Kirill A. Shutemov
2020-02-13 13:59   ` Kirill A. Shutemov
2020-02-13 14:34     ` Matthew Wilcox
2020-02-12  4:18 ` [PATCH v2 06/25] mm: Allow hpages to be arbitrary order Matthew Wilcox
2020-02-13 14:11   ` Kirill A. Shutemov
2020-02-13 14:30     ` Matthew Wilcox
2020-02-12  4:18 ` [PATCH v2 07/25] mm: Introduce thp_size Matthew Wilcox
2020-02-13 14:19   ` Kirill A. Shutemov
2020-02-12  4:18 ` [PATCH v2 08/25] mm: Introduce thp_order Matthew Wilcox
2020-02-13 14:20   ` Kirill A. Shutemov
2020-02-12  4:18 ` [PATCH v2 09/25] fs: Add a filesystem flag for large pages Matthew Wilcox
2020-02-12  7:43   ` Christoph Hellwig
2020-02-12 14:59     ` Matthew Wilcox
2020-02-12  4:18 ` [PATCH v2 10/25] fs: Introduce i_blocks_per_page Matthew Wilcox
2020-02-12  7:44   ` Christoph Hellwig
2020-02-12 15:05     ` Matthew Wilcox
2020-02-12 17:54       ` Christoph Hellwig
2020-02-13 15:40   ` Kirill A. Shutemov
2020-02-13 16:07     ` Matthew Wilcox
2020-02-12  4:18 ` [PATCH v2 11/25] fs: Make page_mkwrite_check_truncate thp-aware Matthew Wilcox
2020-02-13 15:44   ` Kirill A. Shutemov
2020-02-13 16:26     ` Matthew Wilcox
2020-02-12  4:18 ` [PATCH v2 12/25] mm: Add file_offset_of_ helpers Matthew Wilcox
2020-02-12  7:46   ` Christoph Hellwig
2020-02-12  4:18 ` [PATCH v2 13/25] fs: Add zero_user_large Matthew Wilcox
2020-02-14 13:52   ` Kirill A. Shutemov
2020-02-14 16:03     ` Matthew Wilcox
2020-02-18 14:16       ` Kirill A. Shutemov
2020-02-18 16:13         ` Matthew Wilcox
2020-02-18 17:10           ` Kirill A. Shutemov
2020-02-18 18:07             ` Matthew Wilcox
2020-02-21 12:42               ` Kirill A. Shutemov
2020-02-12  4:18 ` [PATCH v2 14/25] iomap: Support arbitrarily many blocks per page Matthew Wilcox
2020-02-12  8:05   ` Christoph Hellwig
2020-02-12  4:18 ` [PATCH v2 15/25] iomap: Support large pages in iomap_adjust_read_range Matthew Wilcox
2020-02-12  8:11   ` Christoph Hellwig
2020-02-12  4:18 ` [PATCH v2 16/25] iomap: Support large pages in read paths Matthew Wilcox
2020-02-12  8:13   ` Christoph Hellwig
2020-02-12 17:45     ` Matthew Wilcox
2020-02-12  4:18 ` [PATCH v2 17/25] iomap: Support large pages in write paths Matthew Wilcox
2020-02-12  8:17   ` Christoph Hellwig
2020-02-12  4:18 ` [PATCH v2 18/25] iomap: Inline data shouldn't see large pages Matthew Wilcox
2020-02-12  8:05   ` Christoph Hellwig
2020-02-12  4:18 ` [PATCH v2 19/25] xfs: Support " Matthew Wilcox
2020-02-12  4:18 ` [PATCH v2 20/25] mm: Make prep_transhuge_page return its argument Matthew Wilcox
2020-02-12  4:18 ` [PATCH v2 21/25] mm: Add __page_cache_alloc_order Matthew Wilcox
2020-02-12  4:18 ` [PATCH v2 22/25] mm: Allow large pages to be added to the page cache Matthew Wilcox
2020-02-12  4:18 ` [PATCH v2 23/25] mm: Allow large pages to be removed from " Matthew Wilcox
2020-02-12  4:18 ` [PATCH v2 24/25] mm: Add large page readahead Matthew Wilcox
2020-02-12  4:18 ` [PATCH v2 25/25] mm: Align THP mappings for non-DAX Matthew Wilcox
2020-02-12  7:50   ` Christoph Hellwig

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