linux-ext4 - Re: [PATCHv6 10/37] filemap: handle huge pages in filemap_fdatawait

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Date:   Thu, 9 Feb 2017 15:03:58 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Jan Kara <jack@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Hugh Dickins <hughd@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-block@...r.kernel.org
Subject: Re: [PATCHv6 10/37] filemap: handle huge pages in
 filemap_fdatawait_range()

On Thu, Jan 26, 2017 at 02:57:52PM +0300, Kirill A. Shutemov wrote:
> @@ -405,9 +405,14 @@ static int __filemap_fdatawait_range(struct address_space *mapping,
>  			if (page->index > end)
>  				continue;
>  
> +			page = compound_head(page);
>  			wait_on_page_writeback(page);
>  			if (TestClearPageError(page))
>  				ret = -EIO;
> +			if (PageTransHuge(page)) {
> +				index = page->index + HPAGE_PMD_NR;
> +				i += index - pvec.pages[i]->index - 1;
> +			}
>  		}

I'm really not sure about your decision to have some interfaces expose
subpages and other expose huge pages.  I think I'd be happier to see
all the existing interfaces made to continue exposing subpages, then
start adding in new interfaces and converting users one at a time
to use them.  For example here, we'd add find_get_huge_pages_tag(),
then pagevec_lookup_huge_tag(), and switch this function from calling
pagevec_lookup_tag() to calling pagevec_lookup_huge_tag() ... then this
function is done; there's no messing around with calling compound_head
or PageTransHuge.

My dream is that eventually all callers will be able to cope with getting
a compound page back from the page cache and we can delete the versions
that return subpages, and rename the 'huge_' variations.