Re: [RFC][PATCH] swsusp: support creating bigger images

Hi,

On Tuesday 25 April 2006 00:16, Pavel Machek wrote:
The appended patch allows swsusp to break the "50% of the normal zone" limit.
This is achieved by using the observation that pages mapped by frozen
userland tasks need not be copied before saving.

I've not followed the patch too carefully, but it is not as bad as I
expected...

With this patch applied I was able to save (and restore ;-)) ~800 MB suspend
images on a box with 1 GB of RAM.

And did it also work second time? ;-).

Yes, somethinkg like 10 times in a row. :-)

Okay, so it can be done, and patch does not look too bad. It still
scares me. Is 800MB image more responsive than 500MB after resume?

Yes, it is, slightly, but I think 800 meg images are impractical for
performance reasons (like IMO everything above 500 meg with the current
hardware). However this means we can save 80% of RAM with the patch
and that should be 400 meg instead of 250 on a 500 meg machine, or
200 meg instead of 125 on a 250 meg machine.

Apart from this we save some memory allocations and copyings during
suspend.

I guess we can revert to old behaviour by simply returning 1 from
need_to_copy, right?

Yes.

I assume that need_to_copy returns 1 in case page is shared by current
and some other process?

Yes. [If the page is shared with the current task it has to mapped by it, in
which case it will be copied.]

[Please don't beat me very hard, just couldn't resist. ;-)]

Well, you are about to force me to learn about mm internals. Plus you
force everyone who tries to modify swsusp. ... it may be okay if
benefit is great enough and if it gets proper testing. Not 2.6.17
material.

Obviously not, and yes it requires a _lot_ of testing especially on boxes
with 512 meg of RAM or less.

Is benefit worth it?

Well, that depends. I think for boxes with 1 GB of RAM or more it's just
unnecessary (as of today, but this may change if faster disks are available).
On boxes with 512 MB of RAM or less it may change a lot as far as the
responsiveness after resume is concerned.

Anyway do you think it may go into -mm (unless Andrew shoots it down,
that is ;-))?

--- linux-2.6.17-rc1-mm3.orig/include/linux/rmap.h 2006-04-22 10:34:33.000000000 +0200
+++ linux-2.6.17-rc1-mm3/include/linux/rmap.h 2006-04-22 10:34:45.000000000 +0200
@@ -104,6 +104,12 @@ pte_t *page_check_address(struct page *,
*/
unsigned long page_address_in_vma(struct page *, struct vm_area_struct *);

+#ifdef CONFIG_SOFTWARE_SUSPEND
+int page_mapped_by_current(struct page *);
+#else
+static inline int page_mapped_by_current(struct page *page) { return 0; }
+#endif /* CONFIG_SOFTWARE_SUSPEND */

I'd leave it undefined. That will prevent nasty surprise when someone
tries to use it w/o CONFIG_SOFTWARE_SUSPEND set.

OK

@@ -251,6 +253,14 @@ int restore_special_mem(void)
return ret;
}

+/* Represents a stacked allocated page to be used in the future */
+struct res_page {
+ struct res_page *next;
+ char padding[PAGE_SIZE - sizeof(void *)];
+};
+
+static struct res_page *page_list;
+
static int pfn_is_nosave(unsigned long pfn)
{
unsigned long nosave_begin_pfn = __pa(&__nosave_begin) >>
PAGE_SHIFT;

Is this part of some other patch?

No. That's because I use a linked list of allocated pages that has been
defined in a slightly different way in
swsusp-use-less-memory-during-resume.patch,
so I just redefine and reuse it.

@@ -490,6 +613,11 @@ void swsusp_free(void)
buffer = NULL;
}

+void swsusp_free(void)
+{
+ free_image();
+ restore_active_inactive_lists();
+}

This still scares me. Nice test would be to
save/restore_active_inactive_lists repeatedly in a loop ... on running
system ... aha, but it probably can't work outside of refrigerator?

restore_active_inactive_lists() only does something if suspend fails
(otherwise the lists active_pages and inactive_pages are empty).

The suspend failure may be simulated by running swapoff and suspendig
with "echo disk > /sys/power/state". However it wouldn't be interesting to
do this in a loop because it will make the same set of pages go out of the
LRU lists and back in a round robin fashion.

I did the the following test:
- ran swapoff
- tried to suspend with "echo disk > /sys/power/state" (which failed,
obviously)
- checked the numbers of active/inactive pages in /proc/meminfo
- ran swapon
- set image_size to 0
- suspended with "echo disk > /sys/power/state" to verify if the right number
of pages would be reclaimed
- resumed

at it worked, apparently.

case SNAPSHOT_UNFREEZE:
if (!data->frozen)
break;
+ if (data->ready) {
+ error = -EPERM;
+ break;
+ }
down(&pm_sem);
thaw_processes();
enable_nonboot_cpus();

Error from UNFREEZE is not nice:

Unfreeze:
unfreeze(snapshot_fd);
return error;
}
... we don't handle it.

It will only occur if we don't call free_snapshot() before unfreeze() which is
a bug anyway. We prevent it from occuring by always calling free_snapshot()
before unfreeze(). :-)

OTOH I guess it will be all fixed on exit()?

Well, sort of, but I think the LRU lists always have to be restored before
thawing processes (eg. kswapd).

Greetings,
Rafael
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