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Date: Wed, 1 Feb 2023 17:51:35 -0500
From: Peter Xu <peterx@...hat.com>
To: Muhammad Usama Anjum <usama.anjum@...labora.com>
Cc: David Hildenbrand <david@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Michał Mirosław <emmir@...gle.com>,
Andrei Vagin <avagin@...il.com>,
Danylo Mocherniuk <mdanylo@...gle.com>,
Paul Gofman <pgofman@...eweavers.com>,
Cyrill Gorcunov <gorcunov@...il.com>,
Alexander Viro <viro@...iv.linux.org.uk>,
Shuah Khan <shuah@...nel.org>,
Christian Brauner <brauner@...nel.org>,
Yang Shi <shy828301@...il.com>,
Vlastimil Babka <vbabka@...e.cz>,
"Liam R . Howlett" <Liam.Howlett@...cle.com>,
Yun Zhou <yun.zhou@...driver.com>,
Suren Baghdasaryan <surenb@...gle.com>,
Alex Sierra <alex.sierra@....com>,
Matthew Wilcox <willy@...radead.org>,
Pasha Tatashin <pasha.tatashin@...een.com>,
Mike Rapoport <rppt@...nel.org>, Nadav Amit <namit@...are.com>,
Axel Rasmussen <axelrasmussen@...gle.com>,
"Gustavo A . R . Silva" <gustavoars@...nel.org>,
Dan Williams <dan.j.williams@...el.com>,
linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
Greg KH <gregkh@...uxfoundation.org>, kernel@...labora.com
Subject: Re: [PATCH v9 1/3] userfaultfd: Add UFFD WP Async support
On Tue, Jan 31, 2023 at 01:32:55PM +0500, Muhammad Usama Anjum wrote:
> Add new WP Async mode (UFFD_FEATURE_WP_ASYNC) which resolves the page
> faults on its own. It can be used to track that which pages have been
> written-to from the time the pages were write-protected. It is very
> efficient way to track the changes as uffd is by nature pte/pmd based.
>
> UFFD synchronous WP sends the page faults to the userspace where the
> pages which have been written-to can be tracked. But it is not efficient.
> This is why this asynchronous version is being added. After setting the
> WP Async, the pages which have been written to can be found in the pagemap
> file or information can be obtained from the PAGEMAP_IOCTL.
>
> Suggested-by: Peter Xu <peterx@...hat.com>
> Signed-off-by: Muhammad Usama Anjum <usama.anjum@...labora.com>
> ---
> Changes in v9:
> - Correct the fault resolution with code contributed by Peter
>
> Changes in v7:
> - Remove UFFDIO_WRITEPROTECT_MODE_ASYNC_WP and add UFFD_FEATURE_WP_ASYNC
> - Handle automatic page fault resolution in better way (thanks to Peter)
>
> update to wp async
> ---
> fs/userfaultfd.c | 11 +++++++++++
> include/linux/userfaultfd_k.h | 6 ++++++
> include/uapi/linux/userfaultfd.h | 8 +++++++-
> mm/memory.c | 23 ++++++++++++++++++++---
> 4 files changed, 44 insertions(+), 4 deletions(-)
>
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index 15a5bf765d43..c17835a0e842 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -1867,6 +1867,10 @@ static int userfaultfd_writeprotect(struct userfaultfd_ctx *ctx,
> mode_wp = uffdio_wp.mode & UFFDIO_WRITEPROTECT_MODE_WP;
> mode_dontwake = uffdio_wp.mode & UFFDIO_WRITEPROTECT_MODE_DONTWAKE;
>
> + /* The unprotection is not supported if in async WP mode */
> + if (!mode_wp && (ctx->features & UFFD_FEATURE_WP_ASYNC))
> + return -EINVAL;
> +
> if (mode_wp && mode_dontwake)
> return -EINVAL;
>
> @@ -1950,6 +1954,13 @@ static int userfaultfd_continue(struct userfaultfd_ctx *ctx, unsigned long arg)
> return ret;
> }
>
> +int userfaultfd_wp_async(struct vm_area_struct *vma)
> +{
> + struct userfaultfd_ctx *ctx = vma->vm_userfaultfd_ctx.ctx;
> +
> + return (ctx && (ctx->features & UFFD_FEATURE_WP_ASYNC));
> +}
> +
> static inline unsigned int uffd_ctx_features(__u64 user_features)
> {
> /*
> diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
> index 9df0b9a762cc..94dcb4dc1b4a 100644
> --- a/include/linux/userfaultfd_k.h
> +++ b/include/linux/userfaultfd_k.h
> @@ -179,6 +179,7 @@ extern int userfaultfd_unmap_prep(struct mm_struct *mm, unsigned long start,
> unsigned long end, struct list_head *uf);
> extern void userfaultfd_unmap_complete(struct mm_struct *mm,
> struct list_head *uf);
> +extern int userfaultfd_wp_async(struct vm_area_struct *vma);
>
> #else /* CONFIG_USERFAULTFD */
>
> @@ -274,6 +275,11 @@ static inline bool uffd_disable_fault_around(struct vm_area_struct *vma)
> return false;
> }
>
> +static inline int userfaultfd_wp_async(struct vm_area_struct *vma)
> +{
> + return false;
> +}
> +
> #endif /* CONFIG_USERFAULTFD */
>
> static inline bool pte_marker_entry_uffd_wp(swp_entry_t entry)
> diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
> index 005e5e306266..f4252ef40071 100644
> --- a/include/uapi/linux/userfaultfd.h
> +++ b/include/uapi/linux/userfaultfd.h
> @@ -38,7 +38,8 @@
> UFFD_FEATURE_MINOR_HUGETLBFS | \
> UFFD_FEATURE_MINOR_SHMEM | \
> UFFD_FEATURE_EXACT_ADDRESS | \
> - UFFD_FEATURE_WP_HUGETLBFS_SHMEM)
> + UFFD_FEATURE_WP_HUGETLBFS_SHMEM | \
> + UFFD_FEATURE_WP_ASYNC)
> #define UFFD_API_IOCTLS \
> ((__u64)1 << _UFFDIO_REGISTER | \
> (__u64)1 << _UFFDIO_UNREGISTER | \
> @@ -203,6 +204,10 @@ struct uffdio_api {
> *
> * UFFD_FEATURE_WP_HUGETLBFS_SHMEM indicates that userfaultfd
> * write-protection mode is supported on both shmem and hugetlbfs.
> + *
> + * UFFD_FEATURE_WP_ASYNC indicates that userfaultfd write-protection
> + * asynchronous mode is supported in which the write fault is automatically
> + * resolved and write-protection is un-set.
Please mention a few other things:
- It only supports anon and shmem (so hugetlb is not supported)
- It will only take effect when any vma is registered with wr-protection
mode. Otherwise the flag will be ignored.
In userfaultfd_register(), we need to fail the ioctl if anyone tries to
register any hugetlb vma with this new flag set.
> */
> #define UFFD_FEATURE_PAGEFAULT_FLAG_WP (1<<0)
> #define UFFD_FEATURE_EVENT_FORK (1<<1)
> @@ -217,6 +222,7 @@ struct uffdio_api {
> #define UFFD_FEATURE_MINOR_SHMEM (1<<10)
> #define UFFD_FEATURE_EXACT_ADDRESS (1<<11)
> #define UFFD_FEATURE_WP_HUGETLBFS_SHMEM (1<<12)
> +#define UFFD_FEATURE_WP_ASYNC (1<<13)
> __u64 features;
>
> __u64 ioctls;
> diff --git a/mm/memory.c b/mm/memory.c
> index 4000e9f017e0..04843e35550e 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3351,8 +3351,21 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
>
> if (likely(!unshare)) {
> if (userfaultfd_pte_wp(vma, *vmf->pte)) {
> - pte_unmap_unlock(vmf->pte, vmf->ptl);
> - return handle_userfault(vmf, VM_UFFD_WP);
> + if (userfaultfd_wp_async(vma)) {
> + /*
> + * Nothing needed (cache flush, TLB invalidations,
> + * etc.) because we're only removing the uffd-wp bit,
> + * which is completely invisible to the user.
> + */
> + pte_t pte = pte_clear_uffd_wp(*vmf->pte);
> +
> + set_pte_at(vma->vm_mm, vmf->address, vmf->pte, pte);
> + /* Update this to be prepared for following up CoW handling */
> + vmf->orig_pte = pte;
> + } else {
> + pte_unmap_unlock(vmf->pte, vmf->ptl);
> + return handle_userfault(vmf, VM_UFFD_WP);
> + }
> }
>
> /*
> @@ -4812,8 +4825,11 @@ static inline vm_fault_t wp_huge_pmd(struct vm_fault *vmf)
>
> if (vma_is_anonymous(vmf->vma)) {
> if (likely(!unshare) &&
> - userfaultfd_huge_pmd_wp(vmf->vma, vmf->orig_pmd))
> + userfaultfd_huge_pmd_wp(vmf->vma, vmf->orig_pmd)) {
> + if (userfaultfd_wp_async(vmf->vma))
> + goto split_and_return;
> return handle_userfault(vmf, VM_UFFD_WP);
> + }
> return do_huge_pmd_wp_page(vmf);
> }
>
> @@ -4825,6 +4841,7 @@ static inline vm_fault_t wp_huge_pmd(struct vm_fault *vmf)
> }
> }
>
> +split_and_return:
The "and_return" is superfluous, IMHO. Just make it "split"?
> /* COW or write-notify handled on pte level: split pmd. */
> __split_huge_pmd(vmf->vma, vmf->pmd, vmf->address, false, NULL);
Would you also update Documentation/admin-guide/mm/userfaultfd.rst in the
same patch?
Thanks,
--
Peter Xu
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