Re: [PATCH 4/4] mvolatile: Add page purging logic & SIGBUS trap

[Minchan Kim]

On Fri, Apr 11, 2014 at 01:15:40PM -0700, John Stultz wrote:
> This patch adds the hooks in the vmscan logic to purge volatile pages
> and mark their pte as purged. With this, volatile pages will be purged
> under pressure, and their ptes swap entry's marked. If the purged pages
> are accessed before being marked non-volatile, we catch this and send a
> SIGBUS.
> 
> This is a simplified implementation that uses logic from Minchan's earlier
> efforts, so credit to Minchan for his work.
> 
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Android Kernel Team <kernel-team@xxxxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Robert Love <rlove@xxxxxxxxxx>
> Cc: Mel Gorman <mel@xxxxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: Dave Hansen <dave@xxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxx>
> Cc: Dmitry Adamushko <dmitry.adamushko@xxxxxxxxx>
> Cc: Neil Brown <neilb@xxxxxxx>
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Cc: Mike Hommey <mh@xxxxxxxxxxxx>
> Cc: Taras Glek <tglek@xxxxxxxxxxx>
> Cc: Jan Kara <jack@xxxxxxx>
> Cc: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxx>
> Cc: Michel Lespinasse <walken@xxxxxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Keith Packard <keithp@xxxxxxxxxx>
> Cc: linux-mm@xxxxxxxxx <linux-mm@xxxxxxxxx>
> Signed-off-by: John Stultz <john.stultz@xxxxxxxxxx>
> ---
>  include/linux/mvolatile.h |   2 +
>  mm/internal.h             |   2 -
>  mm/memory.c               |   8 ++++
>  mm/mvolatile.c            | 120 ++++++++++++++++++++++++++++++++++++++++++++++
>  mm/rmap.c                 |   5 ++
>  mm/vmscan.c               |  12 +++++
>  6 files changed, 147 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/mvolatile.h b/include/linux/mvolatile.h
> index 973bb3b..8cfe6e0 100644
> --- a/include/linux/mvolatile.h
> +++ b/include/linux/mvolatile.h
> @@ -5,4 +5,6 @@
>  
>  #define MVOLATILE_VALID_FLAGS (0) /* Don't yet support any flags */
>  
> +extern int purge_volatile_page(struct page *page);
> +
>  #endif /* _LINUX_MVOLATILE_H */
> diff --git a/mm/internal.h b/mm/internal.h
> index 29e1e76..ea66bf9 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -225,10 +225,8 @@ static inline void mlock_migrate_page(struct page *newpage, struct page *page)
>  
>  extern pmd_t maybe_pmd_mkwrite(pmd_t pmd, struct vm_area_struct *vma);
>  
> -#ifdef CONFIG_TRANSPARENT_HUGEPAGE
>  extern unsigned long vma_address(struct page *page,
>  				 struct vm_area_struct *vma);
> -#endif
>  #else /* !CONFIG_MMU */
>  static inline int mlocked_vma_newpage(struct vm_area_struct *v, struct page *p)
>  {
> diff --git a/mm/memory.c b/mm/memory.c
> index 22dfa61..9043e4c 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -60,6 +60,7 @@
>  #include <linux/migrate.h>
>  #include <linux/string.h>
>  #include <linux/dma-debug.h>
> +#include <linux/mvolatile.h>
>  
>  #include <asm/io.h>
>  #include <asm/pgalloc.h>
> @@ -3643,6 +3644,8 @@ static int handle_pte_fault(struct mm_struct *mm,
>  
>  	entry = *pte;
>  	if (!pte_present(entry)) {
> +		swp_entry_t mvolatile_entry;
> +
>  		if (pte_none(entry)) {
>  			if (vma->vm_ops) {
>  				if (likely(vma->vm_ops->fault))
> @@ -3652,6 +3655,11 @@ static int handle_pte_fault(struct mm_struct *mm,
>  			return do_anonymous_page(mm, vma, address,
>  						 pte, pmd, flags);
>  		}
> +
> +		mvolatile_entry = pte_to_swp_entry(entry);
> +		if (unlikely(is_purged_entry(mvolatile_entry)))
> +			return VM_FAULT_SIGBUS;
> +

There is no pte lock so that is_purged_entry isn't safe so if race happens,
do_swap_page could have a problem so it would be better to handle it
do_swap_page with pte lock because we used swp_pte to indicate purged pte.

I tried to solve it while we were in Napa(you could remember I sent
crap patchset to you privately but failed to fix and I still didn't get
a time to fix it :( ) but I'd like to inform this problem.


>  		if (pte_file(entry))
>  			return do_nonlinear_fault(mm, vma, address,
>  					pte, pmd, flags, entry);
> diff --git a/mm/mvolatile.c b/mm/mvolatile.c
> index 38c8315..16dccee 100644
> --- a/mm/mvolatile.c
> +++ b/mm/mvolatile.c
> @@ -279,3 +279,123 @@ SYSCALL_DEFINE5(mvolatile, unsigned long, start, size_t, len,
>  out:
>  	return ret;
>  }
> +
> +
> +/**
> + * try_to_purge_one - Purge a volatile page from a vma
> + * @page: page to purge
> + * @vma: vma to purge page from
> + *
> + * Finds the pte for a page in a vma, marks the pte as purged
> + * and release the page.
> + */
> +static void try_to_purge_one(struct page *page, struct vm_area_struct *vma)
> +{
> +	struct mm_struct *mm = vma->vm_mm;
> +	pte_t *pte;
> +	pte_t pteval;
> +	spinlock_t *ptl;
> +	unsigned long addr;
> +
> +	VM_BUG_ON(!PageLocked(page));
> +
> +	addr = vma_address(page, vma);
> +	pte = page_check_address(page, mm, addr, &ptl, 0);
> +	if (!pte)
> +		return;
> +
> +	BUG_ON(vma->vm_flags & (VM_SPECIAL|VM_LOCKED|VM_MIXEDMAP|VM_HUGETLB));
> +
> +	flush_cache_page(vma, addr, page_to_pfn(page));
> +	pteval = ptep_clear_flush(vma, addr, pte);
> +
> +	update_hiwater_rss(mm);
> +	if (PageAnon(page))
> +		dec_mm_counter(mm, MM_ANONPAGES);
> +	else
> +		dec_mm_counter(mm, MM_FILEPAGES);
> +
> +	page_remove_rmap(page);
> +	page_cache_release(page);
> +
> +	set_pte_at(mm, addr, pte, swp_entry_to_pte(make_purged_entry()));
> +
> +	pte_unmap_unlock(pte, ptl);
> +	mmu_notifier_invalidate_page(mm, addr);
> +
> +}
> +
> +/**
> + * try_to_purge_vpage - check vma chain and purge from vmas marked volatile
> + * @page: page to purge
> + *
> + * Goes over all the vmas that hold a page, and where the vmas are volatile,
> + * purge the page from the vma.
> + *
> + * Returns 0 on success, -1 on error.
> + */
> +static int try_to_purge_vpage(struct page *page)
> +{
> +	struct anon_vma *anon_vma;
> +	struct anon_vma_chain *avc;
> +	pgoff_t pgoff;
> +	int ret = 0;
> +
> +	anon_vma = page_lock_anon_vma_read(page);
> +	if (!anon_vma)
> +		return -1;
> +
> +	pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
> +	/*
> +	 * During interating the loop, some processes could see a page as
> +	 * purged while others could see a page as not-purged because we have
> +	 * no global lock between parent and child for protecting mvolatile
> +	 * system call during this loop. But it's not a problem because the
> +	 * page is  not *SHARED* page but *COW* page so parent and child can
> +	 * see other data anytime. The worst case by this race is a page was
> +	 * purged but couldn't be discarded so it makes unnecessary pagefault
> +	 * but it wouldn't be severe.
> +	 */
> +	anon_vma_interval_tree_foreach(avc, &anon_vma->rb_root, pgoff, pgoff) {
> +		struct vm_area_struct *vma = avc->vma;
> +
> +		if (!(vma->vm_flags & VM_VOLATILE)) {
> +			ret = -1;
> +			break;
> +		}
> +		try_to_purge_one(page, vma);
> +	}
> +	page_unlock_anon_vma_read(anon_vma);
> +	return ret;
> +}
> +
> +
> +/**
> + * purge_volatile_page - If possible, purge the specified volatile page
> + * @page: page to purge
> + *
> + * Attempts to purge a volatile page, and if needed frees the swap page
> + *
> + * Returns 0 on success, -1 on error.
> + */
> +int purge_volatile_page(struct page *page)
> +{
> +	VM_BUG_ON(!PageLocked(page));
> +	VM_BUG_ON(PageLRU(page));
> +
> +	/* XXX - for now we only support anonymous volatile pages */
> +	if (!PageAnon(page))
> +		return -1;
> +
> +	if (!try_to_purge_vpage(page)) {
> +		if (PageSwapCache(page))
> +			try_to_free_swap(page);
> +
> +		if (page_freeze_refs(page, 1)) {
> +			unlock_page(page);
> +			return 0;
> +		}
> +	}
> +
> +	return -1;
> +}
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 8fc049f..2c2aa7d 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -728,6 +728,11 @@ int page_referenced_one(struct page *page, struct vm_area_struct *vma,
>  				referenced++;
>  		}
>  		pte_unmap_unlock(pte, ptl);
> +		if (vma->vm_flags & VM_VOLATILE) {
> +			pra->mapcount = 0;
> +			pra->vm_flags |= VM_VOLATILE;
> +			return SWAP_FAIL;
> +		}
>  	}
>  
>  	if (referenced) {
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index a9c74b4..0cbfbf6 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -43,6 +43,7 @@
>  #include <linux/sysctl.h>
>  #include <linux/oom.h>
>  #include <linux/prefetch.h>
> +#include <linux/mvolatile.h>
>  
>  #include <asm/tlbflush.h>
>  #include <asm/div64.h>
> @@ -683,6 +684,7 @@ enum page_references {
>  	PAGEREF_RECLAIM,
>  	PAGEREF_RECLAIM_CLEAN,
>  	PAGEREF_KEEP,
> +	PAGEREF_PURGE,
>  	PAGEREF_ACTIVATE,
>  };
>  
> @@ -703,6 +705,13 @@ static enum page_references page_check_references(struct page *page,
>  	if (vm_flags & VM_LOCKED)
>  		return PAGEREF_RECLAIM;
>  
> +	/*
> +	 * If volatile page is reached on LRU's tail, we discard the
> +	 * page without considering recycle the page.
> +	 */
> +	if (vm_flags & VM_VOLATILE)
> +		return PAGEREF_PURGE;
> +
>  	if (referenced_ptes) {
>  		if (PageSwapBacked(page))
>  			return PAGEREF_ACTIVATE;
> @@ -930,6 +939,9 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  		switch (references) {
>  		case PAGEREF_ACTIVATE:
>  			goto activate_locked;
> +		case PAGEREF_PURGE:
> +			if (!purge_volatile_page(page))
> +				goto free_it;
>  		case PAGEREF_KEEP:
>  			goto keep_locked;
>  		case PAGEREF_RECLAIM:
> -- 
> 1.8.3.2
> 
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-- 
Kind regards,
Minchan Kim
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