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Re: [Xen-devel] [BUG] Xen-ballooned memory never returned to domain after partial-free

On 11.12.19 23:08, Nicholas Tsirakis wrote:

The issue I'm seeing is that pages of previously-xenballooned memory are getting
trapped in the balloon on free, specifically when they are free'd in batches
(i.e. not all at once). The first batch is restored to the domain properly, but
subsequent frees are not.

Truthfully I'm not sure if this is a bug or not, but the behavior I'm seeing
doesn't seem to make sense. Note that this "bug" is in the balloon driver, but
the behavior is seen when using the gnttab API, which utilizes the balloon in
the background.


This issue is better illustrated as an example, seen below. Note that the file
in question is drivers/xen/balloon.c:

Kernel version: 4.19.*, code seems consistent on master as well
Relevant configs:

* current_pages = # of pages assigned to domain
* target_pages = # of pages we want assigned to domain
* credit = target - current

Start with current_pages/target_pages = 20 pages

1. alloc 5 pages with gnttab_alloc_pages(). current_pages = 15, credit = 5.
2. alloc 3 pages with gnttab_alloc_pages(). current_pages = 12, credit = 8.
3. some time later, free the last 3 pages with gnttab_free_pages().
4. 3 pages go back to balloon and balloon worker is scheduled since credit > 0.
     * Relevant part of balloon worker shown below:

     do {

         credit = current_credit();

         if (credit > 0) {
             if (balloon_is_inflated())
                 state = increase_reservation(credit);
                 state = reserve_additional_memory();


     } while (credit && state == BP_DONE);

5. credit > 0 and the balloon contains 3 pages, so run increase_reservation. 3
    pages are restored to domain, correctly. current_pages = 15, credit = 5.
6. at this point credit is still > 0, so we loop again.
7. this time, the balloon has 0 pages, so we call reserve_additional_memory,
    seen below. note that CONFIG_XEN_BALLOON_MEMORY_HOTPLUG is disabled, so this
    funciton is very sparse.

     static enum bp_state reserve_additional_memory(void)
         balloon_stats.target_pages = balloon_stats.current_pages;
         return BP_ECANCELED;

8. now target = current = 15, which drops our credit down to 0.

And I think this is the problem. We want here:

    balloon_stats.target_pages = balloon_stats.current_pages +

This should fix it. Thanks for the detailed analysis!

Does the attached patch work for you?

And are you fine with the "Reported-by:" added?


Attachment: 0001-xen-balloon-fix-ballooned-page-accounting-without-ho.patch
Description: Text Data

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