On 04/23/2015 11:03 PM, Kirill A. Shutemov wrote:
We're going to allow mapping of individual 4k pages of THP compound
page. It means we cannot rely on PageTransHuge() check to decide if
map/unmap small page or THP.
The patch adds new argument to rmap functions to indicate whether we want
to operate on whole compound page or only the small page.
Signed-off-by: Kirill A. Shutemov <[email protected]>
Tested-by: Sasha Levin <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
But I wonder about one thing:
-void page_remove_rmap(struct page *page)
+void page_remove_rmap(struct page *page, bool compound)
{
+ int nr = compound ? hpage_nr_pages(page) : 1;
+
if (!PageAnon(page)) {
+ VM_BUG_ON_PAGE(compound && !PageHuge(page), page);
page_remove_file_rmap(page);
return;
}
The function continues by:
/* page still mapped by someone else? */
if (!atomic_add_negative(-1, &page->_mapcount))
return;
/* Hugepages are not counted in NR_ANON_PAGES for now. */
if (unlikely(PageHuge(page)))
return;
The handling of compound parameter for PageHuge() pages feels just
weird. You use hpage_nr_pages() for them which tests PageTransHuge(). It
doesn't break anything and the value of nr is effectively ignored
anyway, but still...
So I wonder, if all callers of page_remove_rmap() for PageHuge() pages
are the two in mm/hugetlb.c, why not just create a special case
function? Or are some callers elsewhere, not aware whether they are
calling this on a PageHuge()? So compound might be even false for those?
If that's all possible and legal, then maybe explain it in a comment to
reduce confusion of further readers. And move the 'nr' assignment to a
place where we are sure it's not a PageHuge(), i.e. right above the
place the value is used, perhaps?
@@ -1181,11 +1191,12 @@ void page_remove_rmap(struct page *page)
* these counters are not modified in interrupt context, and
* pte lock(a spinlock) is held, which implies preemption disabled.
*/
- if (PageTransHuge(page))
+ if (compound) {
+ VM_BUG_ON_PAGE(!PageTransHuge(page), page);
__dec_zone_page_state(page, NR_ANON_TRANSPARENT_HUGEPAGES);
+ }
- __mod_zone_page_state(page_zone(page), NR_ANON_PAGES,
- -hpage_nr_pages(page));
+ __mod_zone_page_state(page_zone(page), NR_ANON_PAGES, -nr);
if (unlikely(PageMlocked(page)))
clear_page_mlock(page);
@@ -1327,7 +1338,7 @@ static int try_to_unmap_one(struct page *page, struct
vm_area_struct *vma,
dec_mm_counter(mm, MM_FILEPAGES);
discard:
- page_remove_rmap(page);
+ page_remove_rmap(page, false);
page_cache_release(page);
out_unmap:
--
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