On Tue, Dec 14, 2021 at 8:56 PM Marco Elver [off-list ref] wrote:

On Mon, Dec 13, 2021 at 10:54PM +0100, andrey.konovalov@linux.dev wrote:
[...]

quoted

+     /*
+      * Skip page_alloc poisoning and zeroing for pages backing VM_ALLOC
+      * mappings. Only effective in HW_TAGS mode.
+      */
+     gfp &= __GFP_SKIP_KASAN_UNPOISON & __GFP_SKIP_ZERO;

This will turn gfp == 0 always. Should it have been

        gfp |= __GFP_SKIP_KASAN_UNPOISON | __GFP_SKIP_ZERO

Oh, this is bad. Thanks for noticing! Will fix in v4.

Also, not sure it matters, but on non-KASAN builds, this will now always
generate an extra instruction. You could conditionally define GFP_SKIP*
only in the KASAN modes that need them, otherwise they become 0, so the
compiler optimizes this out. (Although I think it does does complicate
GFP_SHIFT a little?)

I can implement this, but I don't think a single extra instruction per
vmalloc() matters.

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