On Mon, Mar 11, 2019 at 01:15:55PM +0530, Anshuman Khandual wrote: > Hello Yu, > > We had some disagreements over this series last time around after which I had > posted the following series [1] which tried to enable > ARCH_ENABLE_SPLIT_PMD_PTLOCK > after doing some pgtable accounting changes. After some thoughts and > deliberations > I figure that its better not to do pgtable alloc changes on arm64 creating a > brand > new semantics which ideally should be first debated and agreed upon in > generic MM. > > Though I still see value in a changed generic pgtable page allocation > semantics > for user and kernel space that should not stop us from enabling more granular > PMD level locks through ARCH_ENABLE_SPLIT_PMD_PTLOCK right now. > > [1] https://www.spinics.net/lists/arm-kernel/msg709917.html > > Having said that this series attempts to enable ARCH_ENABLE_SPLIT_PMD_PTLOCK > with > some minimal changes to existing kernel pgtable page allocation code. Hence > just > trying to re-evaluate the series in that isolation. > > On 03/10/2019 06:49 AM, Yu Zhao wrote: > > > For pte page, use pgtable_page_ctor(); for pmd page, use > > pgtable_pmd_page_ctor(); and for the rest (pud, p4d and pgd), > > don't use any. > > This is semantics change. Hence the question is why ? Should not we wait > until a > generic MM agreement in place in this regard ? Can we avoid this ? Is the > change > really required to enable ARCH_ENABLE_SPLIT_PMD_PTLOCK for user space THP > which > this series originally intended to achieve ? > > > > > For now, we don't select ARCH_ENABLE_SPLIT_PMD_PTLOCK and > > pgtable_pmd_page_ctor() is a nop. When we do in patch 3, we > > make sure pmd is not folded so we won't mistakenly call > > pgtable_pmd_page_ctor() on pud or p4d. > > This makes sense from code perspective but I still dont understand the need to > change kernel pgtable page allocation semantics without any real benefit or > fix at > the moment. Cant we keep kernel page table page allocation unchanged for now > and > just enable ARCH_ENABLE_SPLIT_PMD_PTLOCK for user space THP benefits ? Do you > see > any concern with that.
This is not for kernel page tables (i.e. init_mm). This is to accommodate pre-allocated efi_mm page tables because it uses apply_to_page_range() which then calls pte_alloc_map_lock().
