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Date: Fri, 22 Mar 2013 12:34:38 +0200 (EET)
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Dave Hansen <dave@...1.net>
Cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
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: [PATCHv2, RFC 08/30] thp, mm: rewrite add_to_page_cache_locked()
to support huge pages
Dave Hansen wrote:
> On 03/14/2013 10:50 AM, Kirill A. Shutemov wrote:
> > From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
> >
> > For huge page we add to radix tree HPAGE_CACHE_NR pages at once: head
> > page for the specified index and HPAGE_CACHE_NR-1 tail pages for
> > following indexes.
> >
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> > ---
> > mm/filemap.c | 76 ++++++++++++++++++++++++++++++++++++++++------------------
> > 1 file changed, 53 insertions(+), 23 deletions(-)
> >
> > diff --git a/mm/filemap.c b/mm/filemap.c
> > index 2d99191..6bac9e2 100644
> > --- a/mm/filemap.c
> > +++ b/mm/filemap.c
> > @@ -447,6 +447,7 @@ int add_to_page_cache_locked(struct page *page, struct address_space *mapping,
> > pgoff_t offset, gfp_t gfp_mask)
> > {
> > int error;
> > + int nr = 1;
> >
> > VM_BUG_ON(!PageLocked(page));
> > VM_BUG_ON(PageSwapBacked(page));
> > @@ -454,32 +455,61 @@ int add_to_page_cache_locked(struct page *page, struct address_space *mapping,
> > error = mem_cgroup_cache_charge(page, current->mm,
> > gfp_mask & GFP_RECLAIM_MASK);
> > if (error)
> > - goto out;
> > + return error;
> >
> > - error = radix_tree_preload(gfp_mask & ~__GFP_HIGHMEM);
> > - if (error == 0) {
> > - page_cache_get(page);
> > - page->mapping = mapping;
> > - page->index = offset;
> > + if (PageTransHuge(page)) {
> > + BUILD_BUG_ON(HPAGE_CACHE_NR > RADIX_TREE_PRELOAD_NR);
> > + nr = HPAGE_CACHE_NR;
> > + }
>
> That seems like a slightly odd place to put a BUILD_BUG_ON(). I guess
> it doesn't matter to some degree, but does putting it inside the if()
> imply anything?
It actually matters.
HPAGE_CACHE_NR is BUILD_BUG() if !CONFIG_TRANSPARENT_HUGEPAGE, so we need
to hide it inside 'if (PageTransHuge(page))'. PageTransHuge(page) is 0 in
compile time if !CONFIG_TRANSPARENT_HUGEPAGE, so compiler can be smart and
optimize out the check.
> > + error = radix_tree_preload_count(nr, gfp_mask & ~__GFP_HIGHMEM);
> > + if (error) {
> > + mem_cgroup_uncharge_cache_page(page);
> > + return error;
> > + }
> >
> > - spin_lock_irq(&mapping->tree_lock);
> > - error = radix_tree_insert(&mapping->page_tree, offset, page);
> > - if (likely(!error)) {
> > - mapping->nrpages++;
> > - __inc_zone_page_state(page, NR_FILE_PAGES);
> > - spin_unlock_irq(&mapping->tree_lock);
> > - trace_mm_filemap_add_to_page_cache(page);
> > - } else {
> > - page->mapping = NULL;
> > - /* Leave page->index set: truncation relies upon it */
> > - spin_unlock_irq(&mapping->tree_lock);
> > - mem_cgroup_uncharge_cache_page(page);
> > - page_cache_release(page);
>
> I do really like how this rewrite de-indents this code. :)
:)
> > + page_cache_get(page);
> > + spin_lock_irq(&mapping->tree_lock);
> > + page->mapping = mapping;
> > + page->index = offset;
> > + error = radix_tree_insert(&mapping->page_tree, offset, page);
> > + if (unlikely(error))
> > + goto err;
> > + if (PageTransHuge(page)) {
> > + int i;
> > + for (i = 1; i < HPAGE_CACHE_NR; i++) {
> > + page_cache_get(page + i);
> > + page[i].index = offset + i;
>
> Is it OK to leave page->mapping unset for these?
Good catch, thanks.
Seems nobody really use it, since I haven't got any oops, but we need to
set it anyway.
> > + error = radix_tree_insert(&mapping->page_tree,
> > + offset + i, page + i);
> > + if (error) {
> > + page_cache_release(page + i);
> > + break;
> > + }
> > }
>
> Throughout all this new code, I'd really challenge you to try as much as
> possible to minimize the code stuck under "if (PageTransHuge(page))".
I put thp-related code under the 'if' intentionally to be able to optimize
it out if !CONFIG_TRANSPARENT_HUGEPAGE. The config option is disabled by
default.
> For instance, could you change the for() loop a bit and have it shared
> between both cases, like:
>
> > + for (i = 0; i < nr; i++) {
> > + page_cache_get(page + i);
> > + page[i].index = offset + i;
> > + error = radix_tree_insert(&mapping->page_tree,
> > + offset + i, page + i);
> > + if (error) {
> > + page_cache_release(page + i);
> > + break;
> > + }
> > }
>
> > - radix_tree_preload_end();
> > - } else
> > - mem_cgroup_uncharge_cache_page(page);
> > -out:
> > + if (error) {
> > + error = ENOSPC; /* no space for a huge page */
> > + for (i--; i > 0; i--) {
> > + radix_tree_delete(&mapping->page_tree,
> > + offset + i);
> > + page_cache_release(page + i);
> > + }
> > + radix_tree_delete(&mapping->page_tree, offset);
>
> I wonder if this would look any nicer if you just did all the
> page_cache_get()s for the entire huge page along with the head page, and
> then released them all in one place. I think it might shrink the error
> handling paths here.
>
> > + goto err;
> > + }
> > + }
> > + __mod_zone_page_state(page_zone(page), NR_FILE_PAGES, nr);
> > + mapping->nrpages += nr;
> > + spin_unlock_irq(&mapping->tree_lock);
> > + trace_mm_filemap_add_to_page_cache(page);
>
> Do we need to change the tracing to make sure it notes that these were
> or weren't huge pages?
Hm.. I guess we just need to add page order to the trace.
> > + radix_tree_preload_end();
> > + return 0;
> > +err:
> > + page->mapping = NULL;
> > + /* Leave page->index set: truncation relies upon it */
> > + spin_unlock_irq(&mapping->tree_lock);
> > + radix_tree_preload_end();
> > + mem_cgroup_uncharge_cache_page(page);
> > + page_cache_release(page);
> > return error;
> > }
> > EXPORT_SYMBOL(add_to_page_cache_locked);
>
> Does the cgroup code know how to handle these large pages internally
> somehow? It looks like the charge/uncharge is only being done for the
> head page.
It can. We only need to remove PageCompound() check there. Patch is in
git.
--
Kirill A. Shutemov
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