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Date: Mon, 2 Mar 2020 23:59:32 -0800
From: John Hubbard <jhubbard@...dia.com>
To: Claudio Imbrenda <imbrenda@...ux.ibm.com>,
<linux-next@...r.kernel.org>, <akpm@...ux-foundation.org>,
<jack@...e.cz>, <kirill@...temov.name>
CC: <borntraeger@...ibm.com>, <david@...hat.com>,
<aarcange@...hat.com>, <linux-mm@...ck.org>,
<frankja@...ux.ibm.com>, <sfr@...b.auug.org.au>,
<linux-kernel@...r.kernel.org>, <linux-s390@...r.kernel.org>,
Will Deacon <will@...nel.org>
Subject: Re: [PATCH v2 2/2] mm/gup/writeback: add callbacks for inaccessible
pages
On 3/2/20 4:25 PM, Claudio Imbrenda wrote:
> With the introduction of protected KVM guests on s390 there is now a
> concept of inaccessible pages. These pages need to be made accessible
> before the host can access them.
>
> While cpu accesses will trigger a fault that can be resolved, I/O
> accesses will just fail. We need to add a callback into architecture
> code for places that will do I/O, namely when writeback is started or
> when a page reference is taken.
>
> This is not only to enable paging, file backing etc, it is also
> necessary to protect the host against a malicious user space. For
> example a bad QEMU could simply start direct I/O on such protected
> memory. We do not want userspace to be able to trigger I/O errors and
> thus the logic is "whenever somebody accesses that page (gup) or does
> I/O, make sure that this page can be accessed". When the guest tries
> to access that page we will wait in the page fault handler for
> writeback to have finished and for the page_ref to be the expected
> value.
>
> On s390x the function is not supposed to fail, so it is ok to use a
> WARN_ON on failure. If we ever need some more finegrained handling
> we can tackle this when we know the details.
>
> Signed-off-by: Claudio Imbrenda <imbrenda@...ux.ibm.com>
> Acked-by: Will Deacon <will@...nel.org>
> Reviewed-by: David Hildenbrand <david@...hat.com>
> Reviewed-by: Christian Borntraeger <borntraeger@...ibm.com>
> ---
> include/linux/gfp.h | 6 ++++++
> mm/gup.c | 27 ++++++++++++++++++++++++---
> mm/page-writeback.c | 5 +++++
> 3 files changed, 35 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index e5b817cb86e7..be2754841369 100644
> --- a/include/linux/gfp.h
> +++ b/include/linux/gfp.h
> @@ -485,6 +485,12 @@ static inline void arch_free_page(struct page *page, int order) { }
> #ifndef HAVE_ARCH_ALLOC_PAGE
> static inline void arch_alloc_page(struct page *page, int order) { }
> #endif
> +#ifndef HAVE_ARCH_MAKE_PAGE_ACCESSIBLE
> +static inline int arch_make_page_accessible(struct page *page)
> +{
> + return 0;
> +}
> +#endif
>
> struct page *
> __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order, int preferred_nid,
> diff --git a/mm/gup.c b/mm/gup.c
> index 81a95fbe9901..15c47e0e86f8 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -413,6 +413,7 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
> struct page *page;
> spinlock_t *ptl;
> pte_t *ptep, pte;
> + int ret;
>
> /* FOLL_GET and FOLL_PIN are mutually exclusive. */
> if (WARN_ON_ONCE((flags & (FOLL_PIN | FOLL_GET)) ==
> @@ -471,8 +472,6 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
> if (is_zero_pfn(pte_pfn(pte))) {
> page = pte_page(pte);
> } else {
> - int ret;
> -
> ret = follow_pfn_pte(vma, address, ptep, flags);
> page = ERR_PTR(ret);
> goto out;
> @@ -480,7 +479,6 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
> }
>
> if (flags & FOLL_SPLIT && PageTransCompound(page)) {
> - int ret;
> get_page(page);
> pte_unmap_unlock(ptep, ptl);
> lock_page(page);
> @@ -497,6 +495,19 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
> page = ERR_PTR(-ENOMEM);
> goto out;
> }
> + /*
> + * We need to make the page accessible if we are actually going to
> + * poke at its content (pin), otherwise we can leave it inaccessible.
> + * If we cannot make the page accessible, fail.
> + */
> + if (flags & FOLL_PIN) {
> + ret = arch_make_page_accessible(page);
> + if (ret) {
> + unpin_user_page(page);
> + page = ERR_PTR(ret);
> + goto out;
> + }
> + }
That looks good.
> if (flags & FOLL_TOUCH) {
> if ((flags & FOLL_WRITE) &&
> !pte_dirty(pte) && !PageDirty(page))
> @@ -2162,6 +2173,16 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
>
> VM_BUG_ON_PAGE(compound_head(page) != head, page);
>
> + /*
> + * We need to make the page accessible if we are actually
> + * going to poke at its content (pin), otherwise we can
> + * leave it inaccessible. If the page cannot be made
> + * accessible, fail.
> + */
This part looks good, so these two points are just nits:
That's a little bit of repeating what the code does, in the comments. How about:
/*
* We need to make the page accessible if and only if we are
* going to access its content (the FOLL_PIN case). Please see
* Documentation/core-api/pin_user_pages.rst for details.
*/
> + if ((flags & FOLL_PIN) && arch_make_page_accessible(page)) {
> + unpin_user_page(page);
> + goto pte_unmap;
> + }
Your style earlier in the patch was easier on the reader, why not stay consistent
with that (and with this file, which tends also to do this), so:
if (flags & FOLL_PIN) {
ret = arch_make_page_accessible(page);
if (ret) {
unpin_user_page(page);
goto pte_unmap;
}
}
> SetPageReferenced(page);
> pages[*nr] = page;
> (*nr)++;
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index ab5a3cee8ad3..8384be5a2758 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -2807,6 +2807,11 @@ int __test_set_page_writeback(struct page *page, bool keep_write)
> inc_zone_page_state(page, NR_ZONE_WRITE_PENDING);
> }
> unlock_page_memcg(page);
> + /*
> + * If writeback has been triggered on a page that cannot be made
> + * accessible, it is too late.
> + */
> + WARN_ON(arch_make_page_accessible(page));
I'm not deep enough into this area to know if a) this is correct, and b) if there are any
other places that need arch_make_page_accessible() calls. So I'll rely on other
reviewers to help check on that.
> return ret;
>
> }
>
Anyway, I don't see any problems, and as I said, those documentation and style points are
just nitpicks, not bugs.
thanks,
--
John Hubbard
NVIDIA
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