On Tue, May 12, 2026 at 05:05:54PM -0400, Michael S. Tsirkin wrote: > When post_alloc_hook() needs to zero a page for an explicit > __GFP_ZERO allocation for a user page (user_addr is set), use > folio_zero_user() > instead of kernel_init_pages(). This zeros near the faulting > address last, keeping those cachelines hot for the impending > user access. > > folio_zero_user() is only used for explicit __GFP_ZERO, not for > init_on_alloc. On architectures with virtually-indexed caches > (e.g., ARM), clear_user_highpage() performs per-line cache > operations; using it for init_on_alloc would add overhead that > kernel_init_pages() avoids (the page fault path flushes the > cache at PTE installation time regardless). > > No functional change yet: current callers do not pass __GFP_ZERO > for user pages (they zero at the callsite instead). Subsequent > patches will convert them. > > Signed-off-by: Michael S. Tsirkin <[email protected]> > Assisted-by: Claude:claude-opus-4-6 > --- > mm/page_alloc.c | 17 ++++++++++++++--- > 1 file changed, 14 insertions(+), 3 deletions(-) > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index db387dd6b813..76f39dd026ff 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -1861,9 +1861,20 @@ inline void post_alloc_hook(struct page *page, > unsigned int order, > for (i = 0; i != 1 << order; ++i) > page_kasan_tag_reset(page + i); > } > - /* If memory is still not initialized, initialize it now. */ > - if (init) > - kernel_init_pages(page, 1 << order); > + /* > + * If memory is still not initialized, initialize it now. > + * When __GFP_ZERO was explicitly requested and user_addr is set, > + * use folio_zero_user() which zeros near the faulting address > + * last, keeping those cachelines hot. For init_on_alloc, use > + * kernel_init_pages() to avoid unnecessary cache flush overhead > + * on architectures with virtually-indexed caches. > + */ > + if (init) { > + if ((gfp_flags & __GFP_ZERO) && user_addr != USER_ADDR_NONE) > + folio_zero_user(page_folio(page), user_addr); > + else > + kernel_init_pages(page, 1 << order); > + }
Open question but not necessarily in-scope: Should __GFP_ZERO just be implied if (user_addr != USER_ADDR_NONE)? Putting aside how that's done without introducing another gfp flag (maybe something explicit like `alloc_pages_nozero(...)` ), it seems like a very short jump to just adding __GFP_ZERO to any user-alloc by default. I'd be curious to know how many callers across the system omit __GFP_ZERO when allocating a user-page, and whether there might be scenarios where we subtly miss it (seems unlikely and narrow, but very possibly something a driver could do unintentionally). ~Gregory
