On 19.12.24 17:22, Shakeel Butt wrote:
On Thu, Dec 19, 2024 at 10:55:10AM -0500, Zi Yan wrote:
On 19 Dec 2024, at 10:53, Shakeel Butt wrote:
On Thu, Dec 19, 2024 at 04:47:18PM +0100, David Hildenbrand wrote:
On 19.12.24 16:43, Shakeel Butt wrote:
On Thu, Dec 19, 2024 at 02:05:04PM +0100, David Hildenbrand wrote:
On 23.11.24 00:23, Joanne Koong wrote:
For migrations called in MIGRATE_SYNC mode, skip migrating the folio if
it is under writeback and has the AS_WRITEBACK_INDETERMINATE flag set on its
mapping. If the AS_WRITEBACK_INDETERMINATE flag is set on the mapping, the
writeback may take an indeterminate amount of time to complete, and
waits may get stuck.
Signed-off-by: Joanne Koong <joannelkoong@xxxxxxxxx>
Reviewed-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
---
mm/migrate.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/mm/migrate.c b/mm/migrate.c
index df91248755e4..fe73284e5246 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1260,7 +1260,10 @@ static int migrate_folio_unmap(new_folio_t get_new_folio,
*/
switch (mode) {
case MIGRATE_SYNC:
- break;
+ if (!src->mapping ||
+ !mapping_writeback_indeterminate(src->mapping))
+ break;
+ fallthrough;
default:
rc = -EBUSY;
goto out;
Ehm, doesn't this mean that any fuse user can essentially completely block
CMA allocations, memory compaction, memory hotunplug, memory poisoning... ?!
That sounds very bad.
The page under writeback are already unmovable while they are under
writeback. This patch is only making potentially unrelated tasks to
synchronously wait on writeback completion for such pages which in worst
case can be indefinite. This actually is solving an isolation issue on a
multi-tenant machine.
Are you sure, because I read in the cover letter:
"In the current FUSE writeback design (see commit 3be5a52b30aa ("fuse:
support writable mmap"))), a temp page is allocated for every dirty
page to be written back, the contents of the dirty page are copied over to
the temp page, and the temp page gets handed to the server to write back.
This is done so that writeback may be immediately cleared on the dirty
page,"
Which to me means that they are immediately movable again?
Oh sorry, my mistake, yes this will become an isolation issue with the
removal of the temp page in-between which this series is doing. I think
the tradeoff is between extra memory plus slow write performance versus
temporary unmovable memory.
No, the tradeoff is slow FUSE performance vs whole system slowdown due to
memory fragmentation. AS_WRITEBACK_INDETERMINATE indicates it is not
temporary.
If you check the code just above this patch, this
mapping_writeback_indeterminate() check only happen for pages under
writeback which is a temp state. Anyways, fuse folios should not be
unmovable for their lifetime but only while under writeback which is
same for all fs.
But there, writeback is expected to be a temporary thing, not possibly:
"AS_WRITEBACK_INDETERMINATE", that is a BIG difference.
I'll have to NACK anything that violates ZONE_MOVABLE / ALLOC_CMA
guarantees, and unfortunately, it sounds like this is the case here,
unless I am missing something important.
--
Cheers,
David / dhildenb
