Re: [PATCH v5 13/18] mm: don't allow huge faults for files with pre content watches

[Amir Goldstein <amir73il@gmail.com>]

On Wed, Sep 4, 2024 at 10:29 PM Josef Bacik <josef@toxicpanda.com> wrote:
>
> There's nothing stopping us from supporting this, we could simply pass
> the order into the helper and emit the proper length.  However currently
> there's no tests to validate this works properly, so disable it until
> there's a desire to support this along with the appropriate tests.
>
> Reviewed-by: Christian Brauner <brauner@kernel.org>
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>

Reviewed-by: Amir Goldstein <amir73il@gmail.com>

Thanks,
Amir.

> ---
>  mm/memory.c | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index d10e616d7389..3010bcc5e4f9 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -78,6 +78,7 @@
>  #include <linux/ptrace.h>
>  #include <linux/vmalloc.h>
>  #include <linux/sched/sysctl.h>
> +#include <linux/fsnotify.h>
>
>  #include <trace/events/kmem.h>
>
> @@ -5252,8 +5253,17 @@ static vm_fault_t do_numa_page(struct vm_fault *vmf)
>  static inline vm_fault_t create_huge_pmd(struct vm_fault *vmf)
>  {
>         struct vm_area_struct *vma = vmf->vma;
> +       struct file *file = vma->vm_file;
>         if (vma_is_anonymous(vma))
>                 return do_huge_pmd_anonymous_page(vmf);
> +       /*
> +        * Currently we just emit PAGE_SIZE for our fault events, so don't allow
> +        * a huge fault if we have a pre content watch on this file.  This would
> +        * be trivial to support, but there would need to be tests to ensure
> +        * this works properly and those don't exist currently.
> +        */
> +       if (file && fsnotify_file_has_pre_content_watches(file))
> +               return VM_FAULT_FALLBACK;
>         if (vma->vm_ops->huge_fault)
>                 return vma->vm_ops->huge_fault(vmf, PMD_ORDER);
>         return VM_FAULT_FALLBACK;
> @@ -5263,6 +5273,7 @@ static inline vm_fault_t create_huge_pmd(struct vm_fault *vmf)
>  static inline vm_fault_t wp_huge_pmd(struct vm_fault *vmf)
>  {
>         struct vm_area_struct *vma = vmf->vma;
> +       struct file *file = vma->vm_file;
>         const bool unshare = vmf->flags & FAULT_FLAG_UNSHARE;
>         vm_fault_t ret;
>
> @@ -5277,6 +5288,9 @@ static inline vm_fault_t wp_huge_pmd(struct vm_fault *vmf)
>         }
>
>         if (vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) {
> +               /* See comment in create_huge_pmd. */
> +               if (file && fsnotify_file_has_pre_content_watches(file))
> +                       goto split;
>                 if (vma->vm_ops->huge_fault) {
>                         ret = vma->vm_ops->huge_fault(vmf, PMD_ORDER);
>                         if (!(ret & VM_FAULT_FALLBACK))
> @@ -5296,9 +5310,13 @@ static vm_fault_t create_huge_pud(struct vm_fault *vmf)
>  #if defined(CONFIG_TRANSPARENT_HUGEPAGE) &&                    \
>         defined(CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD)
>         struct vm_area_struct *vma = vmf->vma;
> +       struct file *file = vma->vm_file;
>         /* No support for anonymous transparent PUD pages yet */
>         if (vma_is_anonymous(vma))
>                 return VM_FAULT_FALLBACK;
> +       /* See comment in create_huge_pmd. */
> +       if (file && fsnotify_file_has_pre_content_watches(file))
> +               return VM_FAULT_FALLBACK;
>         if (vma->vm_ops->huge_fault)
>                 return vma->vm_ops->huge_fault(vmf, PUD_ORDER);
>  #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
> @@ -5310,12 +5328,16 @@ static vm_fault_t wp_huge_pud(struct vm_fault *vmf, pud_t orig_pud)
>  #if defined(CONFIG_TRANSPARENT_HUGEPAGE) &&                    \
>         defined(CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD)
>         struct vm_area_struct *vma = vmf->vma;
> +       struct file *file = vma->vm_file;
>         vm_fault_t ret;
>
>         /* No support for anonymous transparent PUD pages yet */
>         if (vma_is_anonymous(vma))
>                 goto split;
>         if (vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) {
> +               /* See comment in create_huge_pmd. */
> +               if (file && fsnotify_file_has_pre_content_watches(file))
> +                       goto split;
>                 if (vma->vm_ops->huge_fault) {
>                         ret = vma->vm_ops->huge_fault(vmf, PUD_ORDER);
>                         if (!(ret & VM_FAULT_FALLBACK))
> --
> 2.43.0
>