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Date: Tue, 21 May 2013 14:14:30 -0700
From: Dave Hansen <dave@...1.net>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
CC: Andrea Arcangeli <aarcange@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Al Viro <viro@...iv.linux.org.uk>,
Hugh Dickins <hughd@...gle.com>,
Wu Fengguang <fengguang.wu@...el.com>, Jan Kara <jack@...e.cz>,
Mel Gorman <mgorman@...e.de>, linux-mm@...ck.org,
Andi Kleen <ak@...ux.intel.com>,
Matthew Wilcox <matthew.r.wilcox@...el.com>,
"Kirill A. Shutemov" <kirill@...temov.name>,
Hillf Danton <dhillf@...il.com>, linux-fsdevel@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCHv4 19/39] thp, mm: allocate huge pages in grab_cache_page_write_begin()
On 05/11/2013 06:23 PM, Kirill A. Shutemov wrote:
> From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
>
> Try to allocate huge page if flags has AOP_FLAG_TRANSHUGE.
Why do we need this flag? When might we set it, and when would we not
set it? What kinds of callers need to check for and act on it?
Some of this, at least, needs to make it in to the comment by the #define.
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -194,6 +194,9 @@ extern int do_huge_pmd_numa_page(struct mm_struct *mm, struct vm_area_struct *vm
> #define HPAGE_CACHE_NR ({ BUILD_BUG(); 0; })
> #define HPAGE_CACHE_INDEX_MASK ({ BUILD_BUG(); 0; })
>
> +#define THP_WRITE_ALLOC ({ BUILD_BUG(); 0; })
> +#define THP_WRITE_ALLOC_FAILED ({ BUILD_BUG(); 0; })
Doesn't this belong in the previous patch?
> #define hpage_nr_pages(x) 1
>
> #define transparent_hugepage_enabled(__vma) 0
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index 2e86251..8feeecc 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -270,8 +270,15 @@ unsigned find_get_pages_contig(struct address_space *mapping, pgoff_t start,
> unsigned find_get_pages_tag(struct address_space *mapping, pgoff_t *index,
> int tag, unsigned int nr_pages, struct page **pages);
>
> -struct page *grab_cache_page_write_begin(struct address_space *mapping,
> +struct page *__grab_cache_page_write_begin(struct address_space *mapping,
> pgoff_t index, unsigned flags);
> +static inline struct page *grab_cache_page_write_begin(
> + struct address_space *mapping, pgoff_t index, unsigned flags)
> +{
> + if (!transparent_hugepage_pagecache() && (flags & AOP_FLAG_TRANSHUGE))
> + return NULL;
> + return __grab_cache_page_write_begin(mapping, index, flags);
> +}
OK, so there's some of the behavior.
Could you also call out why you refactored this code? It seems like
you're trying to optimize for the case where AOP_FLAG_TRANSHUGE isn't
set and where the compiler knows that it isn't set.
Could you talk a little bit about the cases that you're thinking of here?
> /*
> * Returns locked page at given index in given cache, creating it if needed.
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 9ea46a4..e086ef0 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2309,25 +2309,44 @@ EXPORT_SYMBOL(generic_file_direct_write);
> * Find or create a page at the given pagecache position. Return the locked
> * page. This function is specifically for buffered writes.
> */
> -struct page *grab_cache_page_write_begin(struct address_space *mapping,
> - pgoff_t index, unsigned flags)
> +struct page *__grab_cache_page_write_begin(struct address_space *mapping,
> + pgoff_t index, unsigned flags)
> {
> int status;
> gfp_t gfp_mask;
> struct page *page;
> gfp_t gfp_notmask = 0;
> + bool thp = (flags & AOP_FLAG_TRANSHUGE) &&
> + IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE_PAGECACHE);
Instead of 'thp', how about 'must_use_thp'? The flag seems to be a
pretty strong edict rather than a hint, so it should be reflected in the
variables derived from it.
"IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE_PAGECACHE)" has also popped up
enough times in the code that it's probably time to start thinking about
shortening it up. It's a wee bit verbose.
> gfp_mask = mapping_gfp_mask(mapping);
> if (mapping_cap_account_dirty(mapping))
> gfp_mask |= __GFP_WRITE;
> if (flags & AOP_FLAG_NOFS)
> gfp_notmask = __GFP_FS;
> + if (thp) {
> + BUG_ON(index & HPAGE_CACHE_INDEX_MASK);
> + BUG_ON(!(gfp_mask & __GFP_COMP));
> + }
> repeat:
> page = find_lock_page(mapping, index);
> - if (page)
> + if (page) {
> + if (thp && !PageTransHuge(page)) {
> + unlock_page(page);
> + page_cache_release(page);
> + return NULL;
> + }
> goto found;
> + }
>
> - page = __page_cache_alloc(gfp_mask & ~gfp_notmask);
> + if (thp) {
> + page = alloc_pages(gfp_mask & ~gfp_notmask, HPAGE_PMD_ORDER);
> + if (page)
> + count_vm_event(THP_WRITE_ALLOC);
> + else
> + count_vm_event(THP_WRITE_ALLOC_FAILED);
> + } else
> + page = __page_cache_alloc(gfp_mask & ~gfp_notmask);
> if (!page)
> return NULL;
> status = add_to_page_cache_lru(page, mapping, index,
> @@ -2342,7 +2361,7 @@ found:
> wait_for_stable_page(page);
> return page;
> }
> -EXPORT_SYMBOL(grab_cache_page_write_begin);
> +EXPORT_SYMBOL(__grab_cache_page_write_begin);
>
> static ssize_t generic_perform_write(struct file *file,
> struct iov_iter *i, loff_t pos)
>
--
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