On Sun, 9 May 2010, Mel Gorman wrote:
>
> It turns out not to be easy to the preallocating of PUDs, PMDs and PTEs
> move_page_tables() needs. To avoid overallocating, it has to follow the same
> logic as move_page_tables duplicating some code in the process. The ugliest
> aspect of all is passing those pre-allocated pages back into move_page_tables
> where they need to be passed down to such functions as __pte_alloc. It turns
> extremely messy.
Umm. What?
That's crazy talk. I'm not talking about preallocating stuff in order to
pass it in to move_page_tables(). I'm talking about just _creating_ the
dang page tables early - preallocating them IN THE PROCESS VM SPACE.
IOW, a patch like this (this is a pseudo-patch, totally untested, won't
compile, yadda yadda - you need to actually make the people who call
"move_page_tables()" call that prepare function first etc etc)
Yeah, if we care about holes in the page tables, we can certainly copy
more of the move_page_tables() logic, but it certainly doesn't matter for
execve(). This just makes sure that the destination page tables exist
first.
Linus
---
mm/mremap.c | 22 +++++++++++++++++++++-
1 files changed, 21 insertions(+), 1 deletions(-)
diff --git a/mm/mremap.c b/mm/mremap.c
index cde56ee..c14505c 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -128,6 +128,26 @@ static void move_ptes(struct vm_area_struct *vma, pmd_t *old_pmd,
#define LATENCY_LIMIT (64 * PAGE_SIZE)
+/*
+ * Preallocate the page tables, so that we can do the actual move
+ * without any allocations, and thus no error handling etc.
+ */
+int prepare_move_page_tables(struct vm_area_struct *vma,
+ unsigned long old_addr, struct vm_area_struct *new_vma,
+ unsigned long new_addr, unsigned long len)
+{
+ unsigned long end_addr = new_addr + len;
+
+ while (new_addr < end_addr) {
+ pmd_t *new_pmd;
+ new_pmd = alloc_new_pmd(vma->vm_mm, new_addr);
+ if (!new_pmd)
+ return -ENOMEM;
+ new_addr = (new_addr + PMD_SIZE) & PMD_MASK;
+ }
+ return 0;
+}
+
unsigned long move_page_tables(struct vm_area_struct *vma,
unsigned long old_addr, struct vm_area_struct *new_vma,
unsigned long new_addr, unsigned long len)
@@ -147,7 +167,7 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
old_pmd = get_old_pmd(vma->vm_mm, old_addr);
if (!old_pmd)
continue;
- new_pmd = alloc_new_pmd(vma->vm_mm, new_addr);
+ new_pmd = get_old_pmd(vma->vm_mm, new_addr);
if (!new_pmd)
break;
next = (new_addr + PMD_SIZE) & PMD_MASK;
--
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