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Message-Id: <20130523143656.B8B73E0090@blue.fi.intel.com>
Date: Thu, 23 May 2013 17:36:56 +0300 (EEST)
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: [PATCHv4 12/39] thp, mm: rewrite add_to_page_cache_locked() to
support huge pages
Dave Hansen wrote:
> On 05/11/2013 06:23 PM, 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.
>
> The really nice way to do these patches is refactor them, first, with no
> behavior change, in one patch, the introduce the new support in the
> second one.
I've split it into two patches.
> > diff --git a/mm/filemap.c b/mm/filemap.c
> > index 61158ac..b0c7c8c 100644
> > --- a/mm/filemap.c
> > +++ b/mm/filemap.c
> > @@ -460,39 +460,62 @@ int add_to_page_cache_locked(struct page *page, struct address_space *mapping,
> > pgoff_t offset, gfp_t gfp_mask)
> > {
> > int error;
> > + int i, nr;
> >
> > VM_BUG_ON(!PageLocked(page));
> > VM_BUG_ON(PageSwapBacked(page));
> >
> > + /* memory cgroup controller handles thp pages on its side */
> > error = mem_cgroup_cache_charge(page, current->mm,
> > gfp_mask & GFP_RECLAIM_MASK);
> > if (error)
> > - goto out;
> > -
> > - error = radix_tree_preload(gfp_mask & ~__GFP_HIGHMEM);
> > - if (error == 0) {
> > - page_cache_get(page);
> > - page->mapping = mapping;
> > - page->index = offset;
> > + 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);
> > - }
> > - radix_tree_preload_end();
> > - } else
> > + if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE_PAGECACHE)) {
> > + BUILD_BUG_ON(HPAGE_CACHE_NR > RADIX_TREE_PRELOAD_NR);
> > + nr = hpage_nr_pages(page);
> > + } else {
> > + BUG_ON(PageTransHuge(page));
> > + nr = 1;
> > + }
>
> Why can't this just be
>
> nr = hpage_nr_pages(page);
>
> Are you trying to optimize for the THP=y, but THP-pagecache=n case?
Yes, I try to optimize for the case.
> > + if (error)
> > + goto err;
>
> I know it's not a super-common thing in the kernel, but could you call
> this "insert_err" or something?
I've changed it to err_insert.
> > + }
> > + __mod_zone_page_state(page_zone(page), NR_FILE_PAGES, nr);
> > + if (PageTransHuge(page))
> > + __inc_zone_page_state(page, NR_FILE_TRANSPARENT_HUGEPAGES);
> > + mapping->nrpages += nr;
> > + spin_unlock_irq(&mapping->tree_lock);
> > + radix_tree_preload_end();
> > + trace_mm_filemap_add_to_page_cache(page);
> > + return 0;
> > +err:
> > + if (i != 0)
> > + error = -ENOSPC; /* no space for a huge page */
> > + page_cache_release(page + i);
> > + page[i].mapping = NULL;
>
> I guess it's a slight behaviour change (I think it's harmless) but if
> you delay doing the page_cache_get() and page[i].mapping= until after
> the radix tree insertion, you can avoid these two lines.
Hm. I don't think it's safe. The spinlock protects radix-tree against
modification, but find_get_page() can see it just after
radix_tree_insert().
The page is locked and IIUC never uptodate at this point, so nobody will
be able to do much with it, but leave it without valid ->mapping is a bad
idea.
> > + for (i--; i >= 0; i--) {
>
> I kinda glossed over that initial "i--". It might be worth a quick
> comment to call it out.
Okay.
> > + /* Leave page->index set: truncation relies upon it */
> > + page[i].mapping = NULL;
> > + radix_tree_delete(&mapping->page_tree, offset + i);
> > + page_cache_release(page + i);
> > + }
> > + spin_unlock_irq(&mapping->tree_lock);
> > + radix_tree_preload_end();
> > + mem_cgroup_uncharge_cache_page(page);
> > return error;
> > }
>
> FWIW, I think you can move the radix_tree_preload_end() up a bit. I
> guess it won't make any practical difference since you're holding a
> spinlock, but it at least makes the point that you're not depending on
> it any more.
Good point.
> I'm also trying to figure out how and when you'd actually have to unroll
> a partial-huge-page worth of radix_tree_insert(). In the small-page
> case, you can collide with another guy inserting in to the page cache.
> But, can that happen in the _middle_ of a THP?
E.g. if you enable THP after some uptime, the mapping can contain small pages
already.
Or if a process map the file with bad alignement (MAP_FIXED) and touch the
area, it will get small pages.
> Despite my nits, the code still looks correct here, so:
>
> Acked-by: Dave Hansen <dave.hansen@...ux.intel.com>
The incremental diff for the patch is below. I guess it's still valid to
use your ack, right?
diff --git a/mm/filemap.c b/mm/filemap.c
index f643062..d004331 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -492,29 +492,33 @@ int add_to_page_cache_locked(struct page *page, struct address_space *mapping,
error = radix_tree_insert(&mapping->page_tree,
offset + i, page + i);
if (error)
- goto err;
+ goto err_insert;
}
+ radix_tree_preload_end();
__mod_zone_page_state(page_zone(page), NR_FILE_PAGES, nr);
if (PageTransHuge(page))
__inc_zone_page_state(page, NR_FILE_TRANSPARENT_HUGEPAGES);
mapping->nrpages += nr;
spin_unlock_irq(&mapping->tree_lock);
- radix_tree_preload_end();
trace_mm_filemap_add_to_page_cache(page);
return 0;
-err:
+err_insert:
+ radix_tree_preload_end();
if (i != 0)
error = -ENOSPC; /* no space for a huge page */
+
+ /* page[i] was not inserted to tree, handle separately */
page_cache_release(page + i);
page[i].mapping = NULL;
- for (i--; i >= 0; i--) {
+ i--;
+
+ for (; i >= 0; i--) {
/* Leave page->index set: truncation relies upon it */
page[i].mapping = NULL;
radix_tree_delete(&mapping->page_tree, offset + i);
page_cache_release(page + i);
}
spin_unlock_irq(&mapping->tree_lock);
- radix_tree_preload_end();
mem_cgroup_uncharge_cache_page(page);
return error;
}
--
Kirill A. Shutemov
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