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SubjectRe: [6/25] Merge pmdisk and swsusp


Patrick Mochel wrote:

>Sorry about the delay; all the conferences are finally over (for now)
>
>On Mon, 19 Jul 2004, Pavel Machek wrote:
>
>
>
>>Hi!
>>
>>
>>
>>>+static void calc_order(void)
>>>+{
>>>+ int diff;
>>>+ int order;
>>>+
>>>+ order = get_bitmask_order(SUSPEND_PD_PAGES(nr_copy_pages));
>>>+ nr_copy_pages += 1 << order;
>>>+ do {
>>>+ diff = get_bitmask_order(SUSPEND_PD_PAGES(nr_copy_pages)) - order;
>>>+ if (diff) {
>>>+ order += diff;
>>>+ nr_copy_pages += 1 << diff;
>>>+ }
>>>+ } while(diff);
>>>+ pagedir_order = order;
>>>+}
>>>
>>>
>>This code is "interesting". Perhaps at least comment would be good
>>here?
>>
>>
>
>Sure, patch below.
>
>
>
> Pat
>
>diff -Nru a/kernel/power/swsusp.c b/kernel/power/swsusp.c
>--- a/kernel/power/swsusp.c 2004-08-01 20:18:35 -07:00
>+++ b/kernel/power/swsusp.c 2004-08-01 20:18:35 -07:00
>@@ -659,13 +659,38 @@
> }
>
>
>+/**
>+ * calc_order - Determine the order of allocation needed for pagedir_save.
>+ *
>+ * This looks tricky, but is just subtle. Please fix it some time.
>+ * Since there are %nr_copy_pages worth of pages in the snapshot, we need
>+ * to allocate enough contiguous space to hold
>+ * (%nr_copy_pages * sizeof(struct pbe)),
>+ * which has the saved/orig locations of the page..
>+ *
>+ * SUSPEND_PD_PAGES() tells us how many pages we need to hold those
>+ * structures, then we call get_bitmask_order(), which will tell us the
>+ * last bit set in the number, starting with 1. (If we need 30 pages, that
>+ * is 0x0000001e in hex. The last bit is the 5th, which is the order we
>+ * would use to allocate 32 contiguous pages).
>+ *
>+ * Since we also need to save those pages, we add the number of pages that
>+ * we need to nr_copy_pages, and in case of an overflow, do the
>+ * calculation again to update the number of pages needed.
>+ *
>+ * With this model, we will tend to waste a lot of memory if we just cross
>+ * an order boundary. Plus, the higher the order of allocation that we try
>+ * to do, the more likely we are to fail in a low-memory situtation
>+ * (though we're unlikely to get this far in such a case, since swsusp
>+ * requires half of memory to be free anyway).
>+ */
>+
>+
> static void calc_order(void)
> {
>- int diff;
>- int order;
>+ int diff = 0;
>+ int order = 0;
>
>- order = get_bitmask_order(SUSPEND_PD_PAGES(nr_copy_pages));
>- nr_copy_pages += 1 << order;
> do {
> diff = get_bitmask_order(SUSPEND_PD_PAGES(nr_copy_pages)) - order;
> if (diff) {
>@@ -687,7 +712,7 @@
> static int alloc_pagedir(void)
> {
> calc_order();
>- pagedir_save = (suspend_pagedir_t *)__get_free_pages(GFP_ATOMIC | __GFP_COLD,
>+ pagedir_save = (suspend_pagedir_t *)__get_free_pages(GFP_ATOMIC | __GFP_COLD,
> pagedir_order);
> if(!pagedir_save)
> return -ENOMEM;
>-
>
>

Why alloc twice for the saved pages, once in calc_order() and then in
alloc_image_pages() ?


--Mika



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