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Date: Tue, 23 Feb 2016 16:05:38 +0300
From: "Kirill A. Shutemov" <kirill@...temov.name>
To: Dave Hansen <dave.hansen@...el.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
Hugh Dickins <hughd@...gle.com>,
Andrea Arcangeli <aarcange@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Vlastimil Babka <vbabka@...e.cz>,
Christoph Lameter <cl@...two.org>,
Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
Jerome Marchand <jmarchan@...hat.com>,
Yang Shi <yang.shi@...aro.org>,
Sasha Levin <sasha.levin@...cle.com>,
linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCHv2 08/28] mm: postpone page table allocation until
do_set_pte()
On Tue, Feb 16, 2016 at 09:17:17AM -0800, Dave Hansen wrote:
> On 02/16/2016 06:26 AM, Kirill A. Shutemov wrote:
> > On Fri, Feb 12, 2016 at 09:44:41AM -0800, Dave Hansen wrote:
> >> On 02/11/2016 06:21 AM, Kirill A. Shutemov wrote:
> >>> diff --git a/include/linux/mm.h b/include/linux/mm.h
> >>> index ca99c0ecf52e..172f4d8e798d 100644
> >>> --- a/include/linux/mm.h
> >>> +++ b/include/linux/mm.h
> >>> @@ -265,6 +265,7 @@ struct fault_env {
> >>> pmd_t *pmd;
> >>> pte_t *pte;
> >>> spinlock_t *ptl;
> >>> + pgtable_t prealloc_pte;
> >>> };
> >>
> >> If we're going to do this fault_env thing, we need some heavy-duty
> >> comments on what the different fields do and what they mean. We don't
> >> want to get in to a situation where we're doing
> >>
> >> void fault_foo(struct fault_env *fe);..
> >>
> >> and then we change the internals of fault_foo() to manipulate a
> >> different set of fe->* variables, or change assumptions, then have
> >> callers randomly break.
> >>
> >> One _nice_ part of passing all the arguments explicitly is that it makes
> >> you go visit all the call sites and think about how the conventions change.
> >>
> >> It just makes me nervous.
> >>
> >> The semantics of having both a ->pte and ->pmd need to be very clearly
> >> spelled out too, please.
> >
> > I've updated this to:
> >
> > /*
> > * Page fault context: passes though page fault handler instead of endless list
> > * of function arguments.
> > */
> > struct fault_env {
> > struct vm_area_struct *vma; /* Target VMA */
> > unsigned long address; /* Faulting virtual address */
> > unsigned int flags; /* FAULT_FLAG_xxx flags */
> > pmd_t *pmd; /* Pointer to pmd entry matching
> > * the 'address'
> > */
>
> Is this just for huge PMDs, or does it also cover normal PMDs pointing
> to PTE pages?
Any.
> Is it populated every time we're at or below the PMD during a fault?
Yes.
> Is it always valid?
It points to relevant entry. Nothing to say about content of the entry in
general.
> > pte_t *pte; /* Pointer to pte entry matching
> > * the 'address'. NULL if the page
> > * table hasn't been allocated.
> > */
>
> What's the relationship between pmd and pte? Can both be set at the
> same time, etc...?
If pte set, pmd is set too. pmd in this case would point to page table pte
is part of.
It's pretty straight-forward.
>
> > spinlock_t *ptl; /* Page table lock.
> > * Protects pte page table if 'pte'
> > * is not NULL, otherwise pmd.
> > */
>
> Are there any rules for callers when a callee puts a value in here?
Nothing in particular. In most cases we acquire and release ptl in the
same function, with few exceptions: write-protect fault path and
do_set_pte(). That's documented around these functions.
> > pgtable_t prealloc_pte; /* Pre-allocated pte page table.
> > * vm_ops->map_pages() calls
> > * do_set_pte() from atomic context.
> > * do_fault_around() pre-allocates
> > * page table to avoid allocation from
> > * atomic context.
> > */
> > };
>
> Who's responsible for freeing this and when?
do_fault_around() frees the page table if it wasn't used.
> >>> /*
> >>> @@ -559,7 +560,8 @@ static inline pte_t maybe_mkwrite(pte_t pte, struct vm_area_struct *vma)
> >>> return pte;
> >>> }
> >>>
> >>> -void do_set_pte(struct fault_env *fe, struct page *page);
> >>> +int do_set_pte(struct fault_env *fe, struct mem_cgroup *memcg,
> >>> + struct page *page);
> >>> #endif
> >>
> >> I think do_set_pte() might be due for a new name if it's going to be
> >> doing allocations internally.
> >
> > Any suggestions?
>
> alloc_set_pte() is probably fine. Just make it clear early in some
> comments that the allocation is conditional.
Ok.
> >>> diff --git a/mm/filemap.c b/mm/filemap.c
> >>> index 28b3875969a8..ba8150d6dc33 100644
> >>> --- a/mm/filemap.c
> >>> +++ b/mm/filemap.c
> >>> @@ -2146,11 +2146,6 @@ void filemap_map_pages(struct fault_env *fe,
> >>> start_pgoff) {
> >>> if (iter.index > end_pgoff)
> >>> break;
> >>> - fe->pte += iter.index - last_pgoff;
> >>> - fe->address += (iter.index - last_pgoff) << PAGE_SHIFT;
> >>> - last_pgoff = iter.index;
> >>> - if (!pte_none(*fe->pte))
> >>> - goto next;
> >>> repeat:
> >>> page = radix_tree_deref_slot(slot);
> >>> if (unlikely(!page))
> >>> @@ -2187,7 +2182,17 @@ repeat:
> >>>
> >>> if (file->f_ra.mmap_miss > 0)
> >>> file->f_ra.mmap_miss--;
> >>> - do_set_pte(fe, page);
> >>> +
> >>> + fe->address += (iter.index - last_pgoff) << PAGE_SHIFT;
> >>> + if (fe->pte)
> >>> + fe->pte += iter.index - last_pgoff;
> >>> + last_pgoff = iter.index;
> >>> + if (do_set_pte(fe, NULL, page)) {
> >>> + /* failed to setup page table: giving up */
> >>> + if (!fe->pte)
> >>> + break;
> >>> + goto unlock;
> >>> + }
> >>
> >> What's the failure here, though?
> >
> > At this point in patchset it never fails: allocation failure is not
> > possible as we pre-allocate page table for faularound.
> >
> > Later after do_set_pmd() is introduced, huge page can be mapped here. By
> > us or under us.
> >
> > I'll update comment.
>
> So why check the return value of do_set_pte()? Why can it return nonzero?
Actually, this part is buggy (loops without result). I used to return
VM_FAULT_NOPAGE when huge page is setup, but not anymore.
I'll replace it with this:
diff --git a/mm/filemap.c b/mm/filemap.c
index de3bb308f5a9..5f655220df69 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2269,12 +2269,12 @@ repeat:
if (fe->pte)
fe->pte += iter.index - last_pgoff;
last_pgoff = iter.index;
- if (alloc_set_pte(fe, NULL, page)) {
- /* Huge page is mapped? */
- if (!fe->pte)
- break;
- goto unlock;
- }
+ alloc_set_pte(fe, NULL, page);
+ /* Huge page is mapped? No need to proceed. */
+ if (pmd_trans_huge(*fe->pmd))
+ break;
+ /* Failed to setup page table? */
+ VM_BUG_ON(!fe->pte);
unlock_page(page);
goto next;
unlock:
--
Kirill A. Shutemov
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