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SubjectRe: [PATCH 0/6] freepgt: free_pgtables shakeup
David S. Miller wrote:
> On Wed, 23 Mar 2005 17:10:15 +0000 (GMT)
> Hugh Dickins <hugh@veritas.com> wrote:
>
>
>>Here's the recut of those patches, including David Miller's vital fixes.
>>I'm addressing these to Nick rather than Andrew, because they're perhaps
>>not fit for -mm until more testing done and the x86_64 32-bit vdso issue
>>handled. I'm unlikely to be responsive until next week, sorry: over to
>>you, Nick - thanks.
>
>
> Works perfectly fine on sparc64.
>

OK, attached is my first cut at slimming down the boundary tests.
I have only had a chance to try it on i386, so I hate to drop it
on you like this - but I *have* put a bit of thought into it....
Treat it as an RFC, and I'll try to test it on a wider range of
things in the next couple of days.
Not that there is anything really nasty with your system David,
so I don't think it will be a big disaster if I can't get this to
work.

Goes on top of Hugh's 6 patches.
Index: linux-2.6/mm/memory.c
===================================================================
--- linux-2.6.orig/mm/memory.c 2005-03-24 10:43:31.000000000 +1100
+++ linux-2.6/mm/memory.c 2005-03-24 11:22:21.000000000 +1100
@@ -139,14 +139,9 @@ static inline void free_pmd_range(struct
start &= PUD_MASK;
if (start < floor)
return;
- if (ceiling) {
- ceiling &= PUD_MASK;
- if (!ceiling)
- return;
- }
- if (end - 1 > ceiling - 1)
+ end = (end + PUD_SIZE - 1) & PUD_MASK;
+ if (end - ceiling - 1 < PUD_SIZE - 1)
return;
-
pmd = pmd_offset(pud, start);
pud_clear(pud);
pmd_free_tlb(tlb, pmd);
@@ -172,14 +167,9 @@ static inline void free_pud_range(struct
start &= PGDIR_MASK;
if (start < floor)
return;
- if (ceiling) {
- ceiling &= PGDIR_MASK;
- if (!ceiling)
- return;
- }
- if (end - 1 > ceiling - 1)
+ end = (end + PGDIR_SIZE - 1) & PGDIR_MASK;
+ if (end - ceiling - 1 < PGDIR_SIZE - 1)
return;
-
pud = pud_offset(pgd, start);
pgd_clear(pgd);
pud_free_tlb(tlb, pud);
@@ -198,6 +188,10 @@ void free_pgd_range(struct mmu_gather **
unsigned long next;
unsigned long start;

+ BUG_ON(addr >= end);
+ /* Don't want end to be 0 and ceiling to be greater than 0-PGDIR_SIZE */
+ BUG_ON(end - 1 > ceiling - 1);
+
/*
* The next few lines have given us lots of grief...
*
@@ -205,23 +199,22 @@ void free_pgd_range(struct mmu_gather **
* there will be no work to do at all, and we'd prefer not to
* go all the way down to the bottom just to discover that.
*
- * Why all these "- 1"s? Because 0 represents both the bottom
- * of the address space and the top of it (using -1 for the
- * top wouldn't help much: the masks would do the wrong thing).
- * The rule is that addr 0 and floor 0 refer to the bottom of
- * the address space, but end 0 and ceiling 0 refer to the top
- * Comparisons need to use "end - 1" and "ceiling - 1" (though
- * that end 0 case should be mythical).
- *
- * Wherever addr is brought up or ceiling brought down, we must
- * be careful to reject "the opposite 0" before it confuses the
- * subsequent tests. But what about where end is brought down
- * by PMD_SIZE below? no, end can't go down to 0 there.
+ * The tricky part of this logic (and similar in free_p?d_range above)
+ * is the 'end' handling. end and ceiling are *exclusive* boundaries,
+ * so their maximum is 0. This suggests the use of two's complement
+ * difference when comparing them, so the wrapping is handled for us.
*
- * Whereas we round start (addr) and ceiling down, by different
- * masks at different levels, in order to test whether a table
- * now has no other vmas using it, so can be freed, we don't
- * bother to round floor or end up - the tests don't need that.
+ * The method is:
+ * - Round end up to the nearest PMD aligned boundary.
+ * - If end has exceeded ceiling, then end - ceiling will be less than
+ * PMD_SIZE.
+ * end can't have approached ceiling from above if ceiling is 0,
+ * because it is rounded up to the next PMD aligned boundary, so
+ * either it will be 0, or 0+PMD_SIZE.
+ * - In the above case that end is 0, or any other time end might be
+ * equal to ceiling, end - ceiling = 0 < PMD_SIZE. So the actual test
+ * we use is (unsigned) end - ceiling - 1 < PMD_SIZE - 1,
+ * to catch this case.
*/

addr &= PMD_MASK;
@@ -230,14 +223,10 @@ void free_pgd_range(struct mmu_gather **
if (!addr)
return;
}
- if (ceiling) {
- ceiling &= PMD_MASK;
- if (!ceiling)
- return;
- }
- if (end - 1 > ceiling - 1)
+ end = (end + PMD_SIZE - 1) & PMD_MASK;
+ if (end - ceiling - 1 < PMD_SIZE - 1)
end -= PMD_SIZE;
- if (addr > end - 1)
+ if (addr >= end)
return;

start = addr;
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