[patch 02/10] Reinstate ZERO_PAGE optimization in get_user_pages() and fix XIP

Greg KH
2008-06-23T23:10:00Z

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2.6.25.9-stable review patch. If anyone has any objections, please let
us know.

------------------
From: Linus Torvalds <torvalds@linux-foundation.org>

commit 89f5b7da2a6bad2e84670422ab8192382a5aeb9f upstream

KAMEZAWA Hiroyuki and Oleg Nesterov point out that since the commit
557ed1fa2620dc119adb86b34c614e152a629a80 ("remove ZERO_PAGE") removed
the ZERO_PAGE from the VM mappings, any users of get_user_pages() will
generally now populate the VM with real empty pages needlessly.

We used to get the ZERO_PAGE when we did the "handle_mm_fault()", but
since fault handling no longer uses ZERO_PAGE for new anonymous pages,
we now need to handle that special case in follow_page() instead.

In particular, the removal of ZERO_PAGE effectively removed the core
file writing optimization where we would skip writing pages that had not
been populated at all, and increased memory pressure a lot by allocating
all those useless newly zeroed pages.

This reinstates the optimization by making the unmapped PTE case the
same as for a non-existent page table, which already did this correctly.

While at it, this also fixes the XIP case for follow_page(), where the
caller could not differentiate between the case of a page that simply
could not be used (because it had no "struct page" associated with it)
and a page that just wasn't mapped.

We do that by simply returning an error pointer for pages that could not
be turned into a "struct page *". The error is arbitrarily picked to be
EFAULT, since that was what get_user_pages() already used for the
equivalent IO-mapped page case.

[ Also removed an impossible test for pte_offset_map_lock() failing:
that's not how that function works ]

Acked-by: Oleg Nesterov <oleg@tv-sign.ru>
Acked-by: Nick Piggin <npiggin@suse.de>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Hugh Dickins <hugh@veritas.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
arch/powerpc/kernel/vdso.c | 2 +-
mm/memory.c | 17 +++++++++++++----
mm/migrate.c | 10 ++++++++++
3 files changed, 24 insertions(+), 5 deletions(-)

--- a/arch/powerpc/kernel/vdso.c
+++ b/arch/powerpc/kernel/vdso.c
@@ -141,7 +141,7 @@ static void dump_one_vdso_page(struct pa
printk("kpg: %p (c:%d,f:%08lx)", __va(page_to_pfn(pg) << PAGE_SHIFT),
page_count(pg),
pg->flags);
- if (upg/* && pg != upg*/) {
+ if (upg && !IS_ERR(upg) /* && pg != upg*/) {
printk(" upg: %p (c:%d,f:%08lx)", __va(page_to_pfn(upg)
<< PAGE_SHIFT),
page_count(upg),
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -943,17 +943,15 @@ struct page *follow_page(struct vm_area_
}

ptep = pte_offset_map_lock(mm, pmd, address, &ptl);
- if (!ptep)
- goto out;

pte = *ptep;
if (!pte_present(pte))
- goto unlock;
+ goto no_page;
if ((flags & FOLL_WRITE) && !pte_write(pte))
goto unlock;
page = vm_normal_page(vma, address, pte);
if (unlikely(!page))
- goto unlock;
+ goto bad_page;

if (flags & FOLL_GET)
get_page(page);
@@ -968,6 +966,15 @@ unlock:
out:
return page;

+bad_page:
+ pte_unmap_unlock(ptep, ptl);
+ return ERR_PTR(-EFAULT);
+
+no_page:
+ pte_unmap_unlock(ptep, ptl);
+ if (!pte_none(pte))
+ return page;
+ /* Fall through to ZERO_PAGE handling */
no_page_table:
/*
* When core dumping an enormous anonymous area that nobody
@@ -1104,6 +1111,8 @@ int get_user_pages(struct task_struct *t

cond_resched();
}
+ if (IS_ERR(page))
+ return i ? i : PTR_ERR(page);
if (pages) {
pages[i] = page;

--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -858,6 +858,11 @@ static int do_move_pages(struct mm_struc
goto set_status;

page = follow_page(vma, pp->addr, FOLL_GET);
+
+ err = PTR_ERR(page);
+ if (IS_ERR(page))
+ goto set_status;
+
err = -ENOENT;
if (!page)
goto set_status;
@@ -921,6 +926,11 @@ static int do_pages_stat(struct mm_struc
goto set_status;

page = follow_page(vma, pm->addr, 0);
+
+ err = PTR_ERR(page);
+ if (IS_ERR(page))
+ goto set_status;
+
err = -ENOENT;
/* Use PageReserved to check for zero page */
if (!page || PageReserved(page))

--