Re: [PATCH 3/4] MADV_VOLATILE: Add purged page detection on setting memory non-volatile

[Minchan Kim <minchan@kernel.org>]

On Tue, Apr 29, 2014 at 02:21:22PM -0700, John Stultz wrote:
> Users of volatile ranges will need to know if memory was discarded.
> This patch adds the purged state tracking required to inform userland
> when it marks memory as non-volatile that some memory in that range
> was purged and needs to be regenerated.
> 
> This simplified implementation which uses some of the logic from
> Minchan's earlier efforts, so credit to Minchan for his work.
> 
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Android Kernel Team <kernel-team@android.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Robert Love <rlove@google.com>
> Cc: Mel Gorman <mel@csn.ul.ie>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Dave Hansen <dave@sr71.net>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: Dmitry Adamushko <dmitry.adamushko@gmail.com>
> Cc: Neil Brown <neilb@suse.de>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Cc: Mike Hommey <mh@glandium.org>
> Cc: Taras Glek <tglek@mozilla.com>
> Cc: Jan Kara <jack@suse.cz>
> Cc: KOSAKI Motohiro <kosaki.motohiro@gmail.com>
> Cc: Michel Lespinasse <walken@google.com>
> Cc: Minchan Kim <minchan@kernel.org>
> Cc: Keith Packard <keithp@keithp.com>
> Cc: linux-mm@kvack.org <linux-mm@kvack.org>
> Acked-by: Jan Kara <jack@suse.cz>
> Signed-off-by: John Stultz <john.stultz@linaro.org>
> ---
>  include/linux/swap.h    |  5 +++
>  include/linux/swapops.h | 10 ++++++
>  mm/mvolatile.c          | 87 +++++++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 102 insertions(+)
> 
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index a32c3da..3abc977 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -55,6 +55,7 @@ enum {
>  	 * 1<<MAX_SWPFILES_SHIFT), so to preserve the values insert
>  	 * new entries here at the top of the enum, not at the bottom
>  	 */
> +	SWP_MVOLATILE_PURGED_NR,
>  #ifdef CONFIG_MEMORY_FAILURE
>  	SWP_HWPOISON_NR,
>  #endif
> @@ -81,6 +82,10 @@ enum {
>  #define SWP_HWPOISON		(MAX_SWAPFILES + SWP_HWPOISON_NR)
>  #endif
>  
> +/*
> + * Purged volatile range pages
> + */
> +#define SWP_MVOLATILE_PURGED	(MAX_SWAPFILES + SWP_MVOLATILE_PURGED_NR)
>  
>  /*
>   * Magic header for a swap area. The first part of the union is
> diff --git a/include/linux/swapops.h b/include/linux/swapops.h
> index c0f7526..fe9c026 100644
> --- a/include/linux/swapops.h
> +++ b/include/linux/swapops.h
> @@ -161,6 +161,16 @@ static inline int is_write_migration_entry(swp_entry_t entry)
>  
>  #endif
>  
> +static inline swp_entry_t make_purged_entry(void)
> +{
> +	return swp_entry(SWP_MVOLATILE_PURGED, 0);
> +}
> +
> +static inline int is_purged_entry(swp_entry_t entry)
> +{
> +	return swp_type(entry) == SWP_MVOLATILE_PURGED;
> +}
> +
>  #ifdef CONFIG_MEMORY_FAILURE
>  /*
>   * Support for hardware poisoned pages
> diff --git a/mm/mvolatile.c b/mm/mvolatile.c
> index edc5894..555d5c4 100644
> --- a/mm/mvolatile.c
> +++ b/mm/mvolatile.c
> @@ -13,8 +13,92 @@
>  #include <linux/mmu_notifier.h>
>  #include <linux/mm_inline.h>
>  #include <linux/mman.h>
> +#include <linux/swap.h>
> +#include <linux/swapops.h>
>  #include "internal.h"
>  
> +struct mvolatile_walker {
> +	struct vm_area_struct *vma;
> +	int page_was_purged;
> +};
> +
> +
> +/**
> + * mvolatile_check_purged_pte - Checks ptes for purged pages
> + * @pmd: pmd to walk
> + * @addr: starting address
> + * @end: end address
> + * @walk: mm_walk ptr (contains ptr to mvolatile_walker)
> + *
> + * Iterates over the ptes in the pmd checking if they have
> + * purged swap entries.
> + *
> + * Sets the mvolatile_walker.page_was_purged to 1 if any were purged,
> + * and clears the purged pte swp entries (since the pages are no
> + * longer volatile, we don't want future accesses to SIGBUS).
> + */
> +static int mvolatile_check_purged_pte(pmd_t *pmd, unsigned long addr,
> +					unsigned long end, struct mm_walk *walk)
> +{
> +	struct mvolatile_walker *vw = walk->private;
> +	pte_t *pte;
> +	spinlock_t *ptl;
> +
> +	if (pmd_trans_huge(*pmd))
> +		return 0;
> +	if (pmd_trans_unstable(pmd))
> +		return 0;
> +
> +	pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);
> +	for (; addr != end; pte++, addr += PAGE_SIZE) {
> +		if (!pte_present(*pte)) {
> +			swp_entry_t mvolatile_entry = pte_to_swp_entry(*pte);
> +
> +			if (unlikely(is_purged_entry(mvolatile_entry))) {
> +
> +				vw->page_was_purged = 1;
> +
> +				/* clear the pte swp entry */
> +				flush_cache_page(vw->vma, addr, pte_pfn(*pte));

Maybe we don't need to flush the cache because there is no mapped page.

> +				ptep_clear_flush(vw->vma, addr, pte);

Maybe we don't need this, either. We didn't set present bit for purged
page but when I look at the internal of ptep_clear_flush, it checks present bit
and skip the TLB flush so it's okay for x86 but not sure other architecture.
More clear function for our purpose would be pte_clear_not_present_full.

And we are changing page table so at least, we need to handle mmu_notifier to
inform that to the client of mmu_notifier.

> +			}
> +		}
> +	}
> +	pte_unmap_unlock(pte - 1, ptl);
> +	cond_resched();
> +
> +	return 0;
> +}
> +
> +
> +/**
> + * mvolatile_check_purged - Sets up a mm_walk to check for purged pages
> + * @vma: ptr to vma we're starting with
> + * @start: start address to walk
> + * @end: end address of walk
> + *
> + * Sets up and calls wa_page_range() to check for purge pages.
> + *
> + * Returns 1 if pages in the range were purged, 0 otherwise.
> + */
> +static int mvolatile_check_purged(struct vm_area_struct *vma,
> +					 unsigned long start,
> +					 unsigned long end)
> +{
> +	struct mvolatile_walker vw;
> +	struct mm_walk mvolatile_walk = {
> +		.pmd_entry = mvolatile_check_purged_pte,
> +		.mm = vma->vm_mm,
> +		.private = &vw,
> +	};
> +	vw.page_was_purged = 0;
> +	vw.vma = vma;
> +
> +	walk_page_range(start, end, &mvolatile_walk);
> +
> +	return vw.page_was_purged;
> +}
> +
>  
>  /**
>   * madvise_volatile - Marks or clears VMAs in the range (start-end) as VM_VOLATILE
> @@ -140,6 +224,9 @@ int madvise_volatile(int mode, unsigned long start, unsigned long end)
>  			break;
>  		vma = vma->vm_next;
>  	}
> +
> +	if (!ret && (mode == MADV_NONVOLATILE))
> +		ret = mvolatile_check_purged(vma, orig_start, end);
>  out:
>  	up_write(&mm->mmap_sem);
>  
> -- 
> 1.9.1
> 
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-- 
Kind regards,
Minchan Kim

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