Re: [RFC v2 PATCH 4/4] mm: pre zero out free pages to speed up page allocation for __GFP_ZERO

On Mon, Jan 4, 2021 at 11:28 AM Matthew Wilcox [off-list ref] wrote:

On Mon, Jan 04, 2021 at 11:19:13AM -0800, Dave Hansen wrote:

quoted

On 12/21/20 8:30 AM, Liang Li wrote:

quoted

--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h

@@ -137,6 +137,9 @@ enum pageflags {
 #endif
 #ifdef CONFIG_64BIT
    PG_arch_2,
+#endif
+#ifdef CONFIG_PREZERO_PAGE
+   PG_zero,
 #endif
    __NR_PAGEFLAGS,

I don't think this is worth a generic page->flags bit.

There's a ton of space in 'struct page' for pages that are in the
allocator.  Can't we use some of that space?

I was going to object to that too, but I think the entire approach is
flawed and needs to be thrown out.  It just nukes the caches in extremely
subtle and hard to measure ways, lowering overall system performance.

At a minimum the performance analysis should at least try to quantify
that externalized cost. Certainly that overhead went somewhere. Maybe
if this overhead was limited to run when the CPU would otherwise go
idle, but that might mean it never runs in practice...