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List-Unsubscribe: On Thu, Sep 18, 2025 at 09:31:30AM +0800, Jinliang Zheng wrote: > On Thu, 18 Sep 2025 01:41:04 +0900, harry.yoo@oracle.com wrote: > > On Wed, Sep 17, 2025 at 11:48:29PM +0800, alexjlzheng@gmail.com wrote: > > > From: Jinliang Zheng > > > > > > After commit 6eb82f994026 ("x86/mm: Pre-allocate P4D/PUD pages for > > > vmalloc area"), we don't need to synchronize kernel mappings in the > > > vmalloc area on x86_64. > > > > Right. > > > > > And commit 58a18fe95e83 ("x86/mm/64: Do not sync vmalloc/ioremap > > > mappings") actually does this. > > > > Right. > > > > > But commit 6659d0279980 ("x86/mm/64: define ARCH_PAGE_TABLE_SYNC_MASK > > > and arch_sync_kernel_mappings()") breaks this. > > > > Good point. > > > > > This patch introduces ARCH_PAGE_TABLE_SYNC_MASK_VMALLOC to avoid > > > unnecessary kernel mappings synchronization of the vmalloc area. > > > > > > Fixes: 6659d0279980 ("x86/mm/64: define ARCH_PAGE_TABLE_SYNC_MASK and arch_sync_kernel_mappings()") > > > > The commit is getting backported to -stable kernels. > > > > Do you think this can cause a visible performance regression from > > user point of view, or it's just a nice optimization to have? > > (and any data to support?) > > Haha, when I woke up in bed this morning, I suddenly realized that I > might have pushed a worthless patch and wasted everyone's precious time. > > Sorry for that. :-( It's okay! > After commit 6eb82f994026 ("x86/mm: Pre-allocate P4D/PUD pages for vmalloc area"), > pgd_alloc_track()/p4d_alloc_track() in vmalloc() and apply_to_range() may should > always return a mask that does not contain PGTBL_PGD_MODIFIED (5 level pgtable) > or PGTBL_P4D_MODIFIED (4 level pgtable), thereby bypassing the call to > arch_sync_kernel_mappings(). Right? Yeah, I was confused about it too ;) I think you're right. because vmalloc area is already populated, p4d_alloc_track() / pud_alloc_track() won't return PGTBL_PGD_MODIFIED or PGTBL_P4D_MODIFIED. > thanks, > Jinliang Zheng. :) > > > > > > Signed-off-by: Jinliang Zheng > > > --- > > > arch/arm/include/asm/page.h | 3 ++- > > > arch/x86/include/asm/pgtable-2level_types.h | 3 ++- > > > arch/x86/include/asm/pgtable-3level_types.h | 3 ++- > > > include/linux/pgtable.h | 4 ++++ > > > mm/memory.c | 2 +- > > > mm/vmalloc.c | 6 +++--- > > > 6 files changed, 14 insertions(+), 7 deletions(-) > > > > > > diff --git a/mm/memory.c b/mm/memory.c > > > index 0ba4f6b71847..cd2488043f8f 100644 > > > --- a/mm/memory.c > > > +++ b/mm/memory.c > > > @@ -3170,7 +3170,7 @@ static int __apply_to_page_range(struct mm_struct *mm, unsigned long addr, > > > break; > > > } while (pgd++, addr = next, addr != end); > > > > > > - if (mask & ARCH_PAGE_TABLE_SYNC_MASK) > > > + if (mask & ARCH_PAGE_TABLE_SYNC_MASK_VMALLOC) > > > arch_sync_kernel_mappings(start, start + size); > > > > But vmalloc is not the only user of apply_to_page_range()? > > > > -- > > Cheers, > > Harry / Hyeonggon -- Cheers, Harry / Hyeonggon