Re: [PATCH 2/9] mm: Add an apply_to_pfn_range interface

[Jerome Glisse]

On Sat, Apr 13, 2019 at 08:34:02AM +0000, Thomas Hellstrom wrote:
> Hi, Jérôme
> 
> On Fri, 2019-04-12 at 17:07 -0400, Jerome Glisse wrote:
> > On Fri, Apr 12, 2019 at 04:04:18PM +0000, Thomas Hellstrom wrote:
> > > This is basically apply_to_page_range with added functionality:
> > > Allocating missing parts of the page table becomes optional, which
> > > means that the function can be guaranteed not to error if
> > > allocation
> > > is disabled. Also passing of the closure struct and callback
> > > function
> > > becomes different and more in line with how things are done
> > > elsewhere.
> > > 
> > > Finally we keep apply_to_page_range as a wrapper around
> > > apply_to_pfn_range
> > > 
> > > The reason for not using the page-walk code is that we want to
> > > perform
> > > the page-walk on vmas pointing to an address space without
> > > requiring the
> > > mmap_sem to be held rather thand on vmas belonging to a process
> > > with the
> > > mmap_sem held.
> > > 
> > > Notable changes since RFC:
> > > Don't export apply_to_pfn range.
> > > 
> > > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > > Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> > > Cc: Will Deacon <will.deacon@xxxxxxx>
> > > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > > Cc: Rik van Riel <riel@xxxxxxxxxxx>
> > > Cc: Minchan Kim <minchan@xxxxxxxxxx>
> > > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > > Cc: Huang Ying <ying.huang@xxxxxxxxx>
> > > Cc: Souptick Joarder <jrdr.linux@xxxxxxxxx>
> > > Cc: "Jérôme Glisse" <jglisse@xxxxxxxxxx>
> > > Cc: linux-mm@xxxxxxxxx
> > > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > > Signed-off-by: Thomas Hellstrom <thellstrom@xxxxxxxxxx>
> > > ---
> > >  include/linux/mm.h |  10 ++++
> > >  mm/memory.c        | 130 ++++++++++++++++++++++++++++++++++-------
> > > ----
> > >  2 files changed, 108 insertions(+), 32 deletions(-)
> > > 
> > > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > > index 80bb6408fe73..b7dd4ddd6efb 100644
> > > --- a/include/linux/mm.h
> > > +++ b/include/linux/mm.h
> > > @@ -2632,6 +2632,16 @@ typedef int (*pte_fn_t)(pte_t *pte,
> > > pgtable_t token, unsigned long addr,
> > >  extern int apply_to_page_range(struct mm_struct *mm, unsigned long
> > > address,
> > >  			       unsigned long size, pte_fn_t fn, void
> > > *data);
> > >  
> > > +struct pfn_range_apply;
> > > +typedef int (*pter_fn_t)(pte_t *pte, pgtable_t token, unsigned
> > > long addr,
> > > +			 struct pfn_range_apply *closure);
> > > +struct pfn_range_apply {
> > > +	struct mm_struct *mm;
> > > +	pter_fn_t ptefn;
> > > +	unsigned int alloc;
> > > +};
> > > +extern int apply_to_pfn_range(struct pfn_range_apply *closure,
> > > +			      unsigned long address, unsigned long
> > > size);
> > >  
> > >  #ifdef CONFIG_PAGE_POISONING
> > >  extern bool page_poisoning_enabled(void);
> > > diff --git a/mm/memory.c b/mm/memory.c
> > > index a95b4a3b1ae2..60d67158964f 100644
> > > --- a/mm/memory.c
> > > +++ b/mm/memory.c
> > > @@ -1938,18 +1938,17 @@ int vm_iomap_memory(struct vm_area_struct
> > > *vma, phys_addr_t start, unsigned long
> > >  }
> > >  EXPORT_SYMBOL(vm_iomap_memory);
> > >  
> > > -static int apply_to_pte_range(struct mm_struct *mm, pmd_t *pmd,
> > > -				     unsigned long addr, unsigned long
> > > end,
> > > -				     pte_fn_t fn, void *data)
> > > +static int apply_to_pte_range(struct pfn_range_apply *closure,
> > > pmd_t *pmd,
> > > +			      unsigned long addr, unsigned long end)
> > >  {
> > >  	pte_t *pte;
> > >  	int err;
> > >  	pgtable_t token;
> > >  	spinlock_t *uninitialized_var(ptl);
> > >  
> > > -	pte = (mm == &init_mm) ?
> > > +	pte = (closure->mm == &init_mm) ?
> > >  		pte_alloc_kernel(pmd, addr) :
> > > -		pte_alloc_map_lock(mm, pmd, addr, &ptl);
> > > +		pte_alloc_map_lock(closure->mm, pmd, addr, &ptl);
> > >  	if (!pte)
> > >  		return -ENOMEM;
> > >  
> > > @@ -1960,86 +1959,107 @@ static int apply_to_pte_range(struct
> > > mm_struct *mm, pmd_t *pmd,
> > >  	token = pmd_pgtable(*pmd);
> > >  
> > >  	do {
> > > -		err = fn(pte++, token, addr, data);
> > > +		err = closure->ptefn(pte++, token, addr, closure);
> > >  		if (err)
> > >  			break;
> > >  	} while (addr += PAGE_SIZE, addr != end);
> > >  
> > >  	arch_leave_lazy_mmu_mode();
> > >  
> > > -	if (mm != &init_mm)
> > > +	if (closure->mm != &init_mm)
> > >  		pte_unmap_unlock(pte-1, ptl);
> > >  	return err;
> > >  }
> > >  
> > > -static int apply_to_pmd_range(struct mm_struct *mm, pud_t *pud,
> > > -				     unsigned long addr, unsigned long
> > > end,
> > > -				     pte_fn_t fn, void *data)
> > > +static int apply_to_pmd_range(struct pfn_range_apply *closure,
> > > pud_t *pud,
> > > +			      unsigned long addr, unsigned long end)
> > >  {
> > >  	pmd_t *pmd;
> > >  	unsigned long next;
> > > -	int err;
> > > +	int err = 0;
> > >  
> > >  	BUG_ON(pud_huge(*pud));
> > >  
> > > -	pmd = pmd_alloc(mm, pud, addr);
> > > +	pmd = pmd_alloc(closure->mm, pud, addr);
> > >  	if (!pmd)
> > >  		return -ENOMEM;
> > > +
> > >  	do {
> > >  		next = pmd_addr_end(addr, end);
> > > -		err = apply_to_pte_range(mm, pmd, addr, next, fn,
> > > data);
> > > +		if (!closure->alloc && pmd_none_or_clear_bad(pmd))
> > > +			continue;
> > > +		err = apply_to_pte_range(closure, pmd, addr, next);
> > >  		if (err)
> > >  			break;
> > >  	} while (pmd++, addr = next, addr != end);
> > >  	return err;
> > >  }
> > >  
> > > -static int apply_to_pud_range(struct mm_struct *mm, p4d_t *p4d,
> > > -				     unsigned long addr, unsigned long
> > > end,
> > > -				     pte_fn_t fn, void *data)
> > > +static int apply_to_pud_range(struct pfn_range_apply *closure,
> > > p4d_t *p4d,
> > > +			      unsigned long addr, unsigned long end)
> > >  {
> > >  	pud_t *pud;
> > >  	unsigned long next;
> > > -	int err;
> > > +	int err = 0;
> > >  
> > > -	pud = pud_alloc(mm, p4d, addr);
> > > +	pud = pud_alloc(closure->mm, p4d, addr);
> > >  	if (!pud)
> > >  		return -ENOMEM;
> > > +
> > >  	do {
> > >  		next = pud_addr_end(addr, end);
> > > -		err = apply_to_pmd_range(mm, pud, addr, next, fn,
> > > data);
> > > +		if (!closure->alloc && pud_none_or_clear_bad(pud))
> > > +			continue;
> > > +		err = apply_to_pmd_range(closure, pud, addr, next);
> > >  		if (err)
> > >  			break;
> > >  	} while (pud++, addr = next, addr != end);
> > >  	return err;
> > >  }
> > >  
> > > -static int apply_to_p4d_range(struct mm_struct *mm, pgd_t *pgd,
> > > -				     unsigned long addr, unsigned long
> > > end,
> > > -				     pte_fn_t fn, void *data)
> > > +static int apply_to_p4d_range(struct pfn_range_apply *closure,
> > > pgd_t *pgd,
> > > +			      unsigned long addr, unsigned long end)
> > >  {
> > >  	p4d_t *p4d;
> > >  	unsigned long next;
> > > -	int err;
> > > +	int err = 0;
> > >  
> > > -	p4d = p4d_alloc(mm, pgd, addr);
> > > +	p4d = p4d_alloc(closure->mm, pgd, addr);
> > >  	if (!p4d)
> > >  		return -ENOMEM;
> > > +
> > >  	do {
> > >  		next = p4d_addr_end(addr, end);
> > > -		err = apply_to_pud_range(mm, p4d, addr, next, fn,
> > > data);
> > > +		if (!closure->alloc && p4d_none_or_clear_bad(p4d))
> > > +			continue;
> > > +		err = apply_to_pud_range(closure, p4d, addr, next);
> > >  		if (err)
> > >  			break;
> > >  	} while (p4d++, addr = next, addr != end);
> > >  	return err;
> > >  }
> > >  
> > > -/*
> > > - * Scan a region of virtual memory, filling in page tables as
> > > necessary
> > > - * and calling a provided function on each leaf page table.
> > > +/**
> > > + * apply_to_pfn_range - Scan a region of virtual memory, calling a
> > > provided
> > > + * function on each leaf page table entry
> > > + * @closure: Details about how to scan and what function to apply
> > > + * @addr: Start virtual address
> > > + * @size: Size of the region
> > > + *
> > > + * If @closure->alloc is set to 1, the function will fill in the
> > > page table
> > > + * as necessary. Otherwise it will skip non-present parts.
> > > + * Note: The caller must ensure that the range does not contain
> > > huge pages.
> > > + * The caller must also assure that the proper mmu_notifier
> > > functions are
> > > + * called. Either in the pte leaf function or before and after the
> > > call to
> > > + * apply_to_pfn_range.
> > 
> > This is wrong there should be a big FAT warning that this can only be
> > use
> > against mmap of device file. The page table walking above is broken
> > for
> > various thing you might find in any other vma like THP, device pte,
> > hugetlbfs,
> 
> I was figuring since we didn't export the function anymore, the warning
> and checks could be left to its users, assuming that any other future
> usage of this function would require mm people audit anyway. But I can
> of course add that warning also to this function if you still want
> that?

Yeah more warning are better, people might start using this, i know
some poeple use unexported symbol and then report bugs while they
just were doing something illegal.

> 
> > ...
> > 
> > Also the mmu notifier can not be call from the pfn callback as that
> > callback
> > happens under page table lock (the change_pte notifier callback is
> > useless
> > and not enough). So it _must_ happen around the call to
> > apply_to_pfn_range
> 
> 
> In the comments I was having in mind usage of, for example
> ptep_clear_flush_notify(). But you're the mmu_notifier expert here. Are
> you saying that function by itself would not be sufficient?
> In that case, should I just scratch the text mentioning the pte leaf
> function?

ptep_clear_flush_notify() is useless ... i have posted patches to either
restore it or remove it. In any case you must call mmu notifier range and
they can not happen under lock. You usage looked fine (in the next patch)
but i would rather have a bit of comment here to make sure people are also
aware of that.

While we can hope that people would cc mm when using mm function, it is
not always the case. So i rather be cautious and warn in comment as much
as possible.

Cheers,
Jérôme