linux-kernel - Re: [PATCH v4 2/4] mm, proc: account for shmem swap in /proc/pid/smaps

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Message-Id: <20151002153702.7bdc4c0483cd9b2ee9e0fba3@linux-foundation.org>
Date:	Fri, 2 Oct 2015 15:37:02 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	linux-mm@...ck.org, Jerome Marchand <jmarchan@...hat.com>,
	Hugh Dickins <hughd@...gle.com>, linux-kernel@...r.kernel.org,
	linux-doc@...r.kernel.org, Michal Hocko <mhocko@...e.cz>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Randy Dunlap <rdunlap@...radead.org>,
	linux-s390@...r.kernel.org,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Linux API <linux-api@...r.kernel.org>,
	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Subject: Re: [PATCH v4 2/4] mm, proc: account for shmem swap in
 /proc/pid/smaps

On Fri,  2 Oct 2015 15:35:49 +0200 Vlastimil Babka <vbabka@...e.cz> wrote:

> Currently, /proc/pid/smaps will always show "Swap: 0 kB" for shmem-backed
> mappings, even if the mapped portion does contain pages that were swapped out.
> This is because unlike private anonymous mappings, shmem does not change pte
> to swap entry, but pte_none when swapping the page out. In the smaps page
> walk, such page thus looks like it was never faulted in.
> 
> This patch changes smaps_pte_entry() to determine the swap status for such
> pte_none entries for shmem mappings, similarly to how mincore_page() does it.
> Swapped out pages are thus accounted for.
> 
> The accounting is arguably still not as precise as for private anonymous
> mappings, since now we will count also pages that the process in question never
> accessed, but only another process populated them and then let them become
> swapped out. I believe it is still less confusing and subtle than not showing
> any swap usage by shmem mappings at all. Also, swapped out pages only becomee a
> performance issue for future accesses, and we cannot predict those for neither
> kind of mapping.
> 
> ...
>
> --- a/include/linux/shmem_fs.h
> +++ b/include/linux/shmem_fs.h
> @@ -60,6 +60,12 @@ extern struct page *shmem_read_mapping_page_gfp(struct address_space *mapping,
>  extern void shmem_truncate_range(struct inode *inode, loff_t start, loff_t end);
>  extern int shmem_unuse(swp_entry_t entry, struct page *page);
>  
> +#ifdef CONFIG_SWAP
> +extern unsigned long shmem_swap_usage(struct inode *inode);
> +extern unsigned long shmem_partial_swap_usage(struct address_space *mapping,
> +						pgoff_t start, pgoff_t end);
> +#endif

CONFIG_SWAP is wrong, isn't it?  It should be CONFIG_SHMEM if anything.

I'd just do

--- a/include/linux/shmem_fs.h~mm-proc-account-for-shmem-swap-in-proc-pid-smaps-fix
+++ a/include/linux/shmem_fs.h
@@ -60,11 +60,9 @@ extern struct page *shmem_read_mapping_p
 extern void shmem_truncate_range(struct inode *inode, loff_t start, loff_t end);
 extern int shmem_unuse(swp_entry_t entry, struct page *page);
 
-#ifdef CONFIG_SWAP
 extern unsigned long shmem_swap_usage(struct inode *inode);
 extern unsigned long shmem_partial_swap_usage(struct address_space *mapping,
 						pgoff_t start, pgoff_t end);
-#endif
 
 static inline struct page *shmem_read_mapping_page(
 				struct address_space *mapping, pgoff_t index)


We don't need the ifdefs around declarations and they're a pain to
maintain and they'd add a *ton* of clutter if we even tried to do this
for real.
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