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Message-ID: <a3c7a668-ecc1-b6ca-202b-8459726b1941@linux.intel.com>
Date: Thu, 23 Jun 2022 12:24:48 +0200
From: Thomas Hellström
<thomas.hellstrom@...ux.intel.com>
To: "Gowans, James" <jgowans@...zon.com>,
"linux-mm@...ck.org" <linux-mm@...ck.org>,
"christian.koenig@....com" <christian.koenig@....com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Schönherr, Jan H. <jschoenh@...zon.de>,
"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [PATCH] mm: Split huge PUD on wp_huge_pud fallback
On 6/23/22 07:24, Gowans, James wrote:
> Currently the implementation will split the PUD when a fallback is taken
> inside the create_huge_pud function. This isn't where it should be done:
> the splitting should be done in wp_huge_pud, just like it's done for
> PMDs. Reason being that if a callback is taken during create, there is
> no PUD yet so nothing to split, whereas if a fallback is taken when
> encountering a write protection fault there is something to split.
>
> It looks like this was the original intention with the commit where the
> splitting was introduced, but somehow it got moved to the wrong place
> between v1 and v2 of the patch series. Rebase mistake perhaps.
>
> Fixes: 327e9fd48972 ("mm: Split huge pages on write-notify or COW")
Some time since I looked into this, but looks correct to me.
Reviewed-by: Thomas Hellström <thomas.hellstrom@...ux.intel.com>
>
> Cc: Thomas Hellström <thomas.hellstrom@...ux.intel.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Christian König <christian.koenig@....com>
> Cc: Jan H. Schönherr <jschoenh@...zon.de>
> Signed-off-by: James Gowans <jgowans@...zon.com>
> ---
> mm/memory.c | 31 ++++++++++++++++---------------
> 1 file changed, 16 insertions(+), 15 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 7a089145cad4..4cf7d4b6c950 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4802,29 +4802,30 @@ static vm_fault_t create_huge_pud(struct vm_fault *vmf)
> defined(CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD)
> /* No support for anonymous transparent PUD pages yet */
> if (vma_is_anonymous(vmf->vma))
> - goto split;
> - if (vmf->vma->vm_ops->huge_fault) {
> - vm_fault_t ret = vmf->vma->vm_ops->huge_fault(vmf, PE_SIZE_PUD);
> -
> - if (!(ret & VM_FAULT_FALLBACK))
> - return ret;
> - }
> -split:
> - /* COW or write-notify not handled on PUD level: split pud.*/
> - __split_huge_pud(vmf->vma, vmf->pud, vmf->address);
> + return VM_FAULT_FALLBACK;
> + if (vmf->vma->vm_ops->huge_fault)
> + return vmf->vma->vm_ops->huge_fault(vmf, PE_SIZE_PUD);
> #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
> return VM_FAULT_FALLBACK;
> }
>
> static vm_fault_t wp_huge_pud(struct vm_fault *vmf, pud_t orig_pud)
> {
> -#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +#if defined(CONFIG_TRANSPARENT_HUGEPAGE) && \
> + defined(CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD)
> /* No support for anonymous transparent PUD pages yet */
> if (vma_is_anonymous(vmf->vma))
> - return VM_FAULT_FALLBACK;
> - if (vmf->vma->vm_ops->huge_fault)
> - return vmf->vma->vm_ops->huge_fault(vmf, PE_SIZE_PUD);
> -#endif /* CONFIG_TRANSPARENT_HUGEPAGE */
> + goto split;
> + if (vmf->vma->vm_ops->huge_fault) {
> + vm_fault_t ret = vmf->vma->vm_ops->huge_fault(vmf, PE_SIZE_PUD);
> +
> + if (!(ret & VM_FAULT_FALLBACK))
> + return ret;
> + }
> +split:
> + /* COW or write-notify not handled on PUD level: split pud.*/
> + __split_huge_pud(vmf->vma, vmf->pud, vmf->address);
> +#endif /* CONFIG_TRANSPARENT_HUGEPAGE && CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD */
> return VM_FAULT_FALLBACK;
> }
>
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