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migration: write zero pages when postcopy enabled

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During multifd migration, zero pages are written if
they are migrated more than once.

This may result in a migration thread hang issue when
multifd and postcopy are enabled together.

When postcopy is enabled, always write zero pages as and
when they are migrated.

Signed-off-by: Prasad Pandit <pjp@fedoraproject.org>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20250512125124.147064-2-ppandit@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
This commit is contained in:
Prasad Pandit 2025-05-12 18:21:22 +05:30 • committed by Peter Xu
parent 371650534d
commit 249543d0c0
1 changed files with 20 additions and 2 deletions
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22
migration/multifd-zero-page.c
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@ -85,9 +85,27 @@ void multifd_recv_zero_page_process(MultiFDRecvParams *p)
{ {
for (int i = 0; i < p->zero_num; i++) { for (int i = 0; i < p->zero_num; i++) {
void *page = p->host + p->zero[i]; void *page = p->host + p->zero[i];
if (ramblock_recv_bitmap_test_byte_offset(p->block, p->zero[i])) { bool received =
ramblock_recv_bitmap_test_byte_offset(p->block, p->zero[i]);
/*
* During multifd migration zero page is written to the memory
* only if it is migrated more than once.
*
* It becomes a problem when both multifd & postcopy options are
* enabled. If the zero page which was skipped during multifd phase,
* is accessed during the postcopy phase of the migration, a page
* fault occurs. But this page fault is not served because the
* 'receivedmap' says the zero page is already received. Thus the
* thread accessing that page may hang.
*
* When postcopy is enabled, always write the zero page as and when
* it is migrated.
*/
if (migrate_postcopy_ram() || received) {
memset(page, 0, multifd_ram_page_size()); memset(page, 0, multifd_ram_page_size());
} else { }
if (!received) {
ramblock_recv_bitmap_set_offset(p->block, p->zero[i]); ramblock_recv_bitmap_set_offset(p->block, p->zero[i]);
} }
} }