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SubjectRe: [PATCH] frontswap: support exclusive gets if tmem backend is capable
On Fri, Aug 31, 2012 at 08:48:10AM -0700, Dan Magenheimer wrote:
> Konrad, can this go in linux-next and in the next window?
>
> Tmem, as originally specified, assumes that "get" operations
> performed on persistent pools never flush the page of data out
> of tmem on a successful get, waiting instead for a flush
> operation. This is intended to mimic the model of a swap
> disk, where a disk read is non-destructive. Unlike a
> disk, however, freeing up the RAM can be valuable. Over
> the years that frontswap was in the review process, several
> reviewers (and notably Hugh Dickins in 2010) pointed out that
> this would result, at least temporarily, in two copies of the
> data in RAM: one (compressed for zcache) copy in tmem,
> and one copy in the swap cache. We wondered if this could
> be done differently, at least optionally.
>
> This patch allows tmem backends to instruct the frontswap
> code that this backend performs exclusive gets. Zcache2
> already contains hooks to support this feature. Other
> backends are completely unaffected unless/until they are
> updated to support this feature.
>
> While it is not clear that exclusive gets are a performance
> win on all workloads at all times, this small patch allows for
> experimentation by backends.
>
> P.S. Let's not quibble about the naming of "get" vs "read" vs
> "load" etc. The naming is currently horribly inconsistent between
> cleancache and frontswap and existing tmem backends, so will need
> to be straightened out as a separate patch. "Get" is used
> by the tmem architecture spec, existing backends, and
> all documentation and presentation material so I am
> using it in this patch.
>
> Signed-off-by: Dan Magenheimer <dan.magenheimer@oracle.com>
>
> diff --git a/include/linux/frontswap.h b/include/linux/frontswap.h
> index 0e4e2ee..3044254 100644
> --- a/include/linux/frontswap.h
> +++ b/include/linux/frontswap.h
> @@ -19,6 +19,8 @@ extern struct frontswap_ops
> extern void frontswap_shrink(unsigned long);
> extern unsigned long frontswap_curr_pages(void);
> extern void frontswap_writethrough(bool);
> +#define FRONTSWAP_HAS_EXCLUSIVE_GETS
> +extern void frontswap_tmem_exclusive_gets(bool);

I don't think you need the #define here..
>
> extern void __frontswap_init(unsigned type);
> extern int __frontswap_store(struct page *page);
> diff --git a/mm/frontswap.c b/mm/frontswap.c
> index e250255..b1496fb 100644
> --- a/mm/frontswap.c
> +++ b/mm/frontswap.c
> @@ -48,6 +48,13 @@ EXPORT_SYMBOL(frontswap_enabled);
> */
> static bool frontswap_writethrough_enabled __read_mostly;
>
> +/*
> + * If enabled, the underlying tmem implementation is capable of doing
> + * exclusive gets, so frontswap_load, on a successful tmem_get must
> + * mark the page as no longer in frontswap AND mark it dirty.
> + */
> +static bool frontswap_tmem_exclusive_gets_enabled __read_mostly;
> +
> #ifdef CONFIG_DEBUG_FS
> /*
> * Counters available via /sys/kernel/debug/frontswap (if debugfs is
> @@ -101,6 +108,15 @@ void frontswap_writethrough(bool enable)
> EXPORT_SYMBOL(frontswap_writethrough);
>
> /*
> + * Enable/disable frontswap exclusive gets (see above).
> + */
> +void frontswap_tmem_exclusive_gets(bool enable)
> +{
> + frontswap_tmem_exclusive_gets_enabled = enable;
> +}
> +EXPORT_SYMBOL(frontswap_tmem_exclusive_gets);

We got two of these now - the writethrough and this one. Merging
them in one function and one flag might be better. So something like:
static int frontswap_mode = 0;

void frontswap_set_mode(int set_mode)
{
if (mode & (FRONTSWAP_WRITETH | FRONTSWAP_EXCLUS..)
mode |= set_mode;
}
... and
> +
> +/*
> * Called when a swap device is swapon'd.
> */
> void __frontswap_init(unsigned type)
> @@ -174,8 +190,13 @@ int __frontswap_load(struct page *page)
> BUG_ON(sis == NULL);
> if (frontswap_test(sis, offset))
> ret = (*frontswap_ops.load)(type, offset, page);
> - if (ret == 0)
> + if (ret == 0) {
> inc_frontswap_loads();
> + if (frontswap_tmem_exclusive_gets_enabled) {

For these perhaps use asm goto for optimization? Is this showing up in
perf as a hotspot? The asm goto might be a bit too much.

> + SetPageDirty(page);
> + frontswap_clear(sis, offset);
> + }
> + }
> return ret;
> }
> EXPORT_SYMBOL(__frontswap_load);


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