[PATCH] mm: Fixup the condition whether the page cache is free

[PATCH] mm: Fixup the condition whether the page cache is free

[Li Haifeng]

When a page cache is to reclaim, we should to decide whether the page
cache is free.
IMO, the condition whether a page cache is free should be 3 in page
frame reclaiming. The reason lists as below.

When page is allocated, the page->_count is 1(code fragment is code-1 ).
And when the page is allocated for reading files from extern disk, the
page->_count will increment 1 by page_cache_get() in
add_to_page_cache_locked()(code fragment is code-2). When the page is to
reclaim, the isolated LRU list also increase the page->_count(code
fragment is code-3).

According above reasons, when the file page is freeable, the
page->_count should be 3 instead of 2.

<code-1>
buffered_rmqueue ->prep_new_page->set_page_refcounted:
24 /*
25  * Turn a non-refcounted page (->_count == 0) into refcounted with
26  * a count of one.
27  */
28 static inline void set_page_refcounted(struct page *page)
29 {
30         VM_BUG_ON(PageTail(page));
31         VM_BUG_ON(atomic_read(&page->_count));
32         set_page_count(page, 1);
33 }

<code-2>
do_generic_file_read ->add_to_page_cache_lru-> add_to_page_cache->
add_to_page_cache_locked:
int add_to_page_cache_locked(struct page *page, struct address_space
*mapping,
                pgoff_t offset, gfp_t gfp_mask)
{
…
           page_cache_get(page);
                page->mapping = mapping;
                page->index = offset;

                spin_lock_irq(&mapping->tree_lock);
                error = radix_tree_insert(&mapping->page_tree, offset,
page);
                if (likely(!error)) {
                        mapping->nrpages++;
                        __inc_zone_page_state(page, NR_FILE_PAGES);
                        spin_unlock_irq(&mapping->tree_lock);
…
}
<code-3>
static noinline_for_stack unsigned long
shrink_inactive_list(unsigned long nr_to_scan, struct mem_cgroup_zone
*mz,
                     struct scan_control *sc, int priority, int file)
{
…
       nr_taken = isolate_lru_pages(nr_to_scan, mz, &page_list,
&nr_scanned,
                                     sc, isolate_mode, 0, file);
…
	   nr_reclaimed = shrink_page_list(&page_list, mz, sc, priority,
                                                &nr_dirty,
&nr_writeback);
}
Remarks for code-3:
isolate_lru_pages() will call get_page_unless_zero() ultimately to
increase the page->_count by 1.
And shrink_page_list() will call is_page_cache_freeable() finally to
check whether the page cache is free.

Re: [PATCH] mm: Fixup the condition whether the page cache is free

[Johannes Weiner]

On Mon, Mar 04, 2013 at 09:54:26AM +0800, Li Haifeng wrote:
> When a page cache is to reclaim, we should to decide whether the page
> cache is free.
> IMO, the condition whether a page cache is free should be 3 in page
> frame reclaiming. The reason lists as below.
> 
> When page is allocated, the page->_count is 1(code fragment is code-1 ).
> And when the page is allocated for reading files from extern disk, the
> page->_count will increment 1 by page_cache_get() in
> add_to_page_cache_locked()(code fragment is code-2). When the page is to
> reclaim, the isolated LRU list also increase the page->_count(code
> fragment is code-3).

The page count is initialized to 1, but that does not stay with the
object.  It's a reference that is passed to the allocating task, which
drops it again when it's done with the page.  I.e. the pattern is like
this:

instantiation:
page = page_cache_alloc()	/* instantiator reference -> 1 */
add_to_page_cache(page, mapping, offset)
  get_page(page)		/* page cache reference -> 2 */
lru_cache_add(page)
  get_page(page)		/* pagevec reference -> 3 */
/* ...initiate read, write, associate buffers, ... */
page_cache_release(page)	/* drop instantiator reference -> 2 + private */

reclaim:
lru_add_drain()
  page_cache_release(page)	/* drop pagevec reference -> 1 + private */
__isolate_lru_page(page)
  page_cache_get(page)		/* reclaim reference -> 2 + private */
is_page_cache_freeable(page)
try_to_free_buffers()		/* drop buffer ref -> 2 */
__remove_mapping()		/* drop page cache and isolator ref -> 0 */
free_hot_cold_page()

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Re: [PATCH] mm: Fixup the condition whether the page cache is free

[Li Haifeng]

Thanks very much for you explanation. :-)

2013/3/4 Johannes Weiner <hannes@cmpxchg.org>:
> On Mon, Mar 04, 2013 at 09:54:26AM +0800, Li Haifeng wrote:
>> When a page cache is to reclaim, we should to decide whether the page
>> cache is free.
>> IMO, the condition whether a page cache is free should be 3 in page
>> frame reclaiming. The reason lists as below.
>>
>> When page is allocated, the page->_count is 1(code fragment is code-1 ).
>> And when the page is allocated for reading files from extern disk, the
>> page->_count will increment 1 by page_cache_get() in
>> add_to_page_cache_locked()(code fragment is code-2). When the page is to
>> reclaim, the isolated LRU list also increase the page->_count(code
>> fragment is code-3).
>
> The page count is initialized to 1, but that does not stay with the
> object.  It's a reference that is passed to the allocating task, which
> drops it again when it's done with the page.  I.e. the pattern is like
> this:
>
> instantiation:
> page = page_cache_alloc()       /* instantiator reference -> 1 */
> add_to_page_cache(page, mapping, offset)
>   get_page(page)                /* page cache reference -> 2 */
> lru_cache_add(page)
>   get_page(page)                /* pagevec reference -> 3 */
> /* ...initiate read, write, associate buffers, ... */
> page_cache_release(page)        /* drop instantiator reference -> 2 + private */
>
> reclaim:
> lru_add_drain()
>   page_cache_release(page)      /* drop pagevec reference -> 1 + private */
> __isolate_lru_page(page)
>   page_cache_get(page)          /* reclaim reference -> 2 + private */
> is_page_cache_freeable(page)
> try_to_free_buffers()           /* drop buffer ref -> 2 */
> __remove_mapping()              /* drop page cache and isolator ref -> 0 */
> free_hot_cold_page()

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Re: [PATCH] mm: Fixup the condition whether the page cache is free

[Simon Jeons]

Hi Johannes,
On 03/04/2013 11:09 PM, Johannes Weiner wrote:
> On Mon, Mar 04, 2013 at 09:54:26AM +0800, Li Haifeng wrote:
>> When a page cache is to reclaim, we should to decide whether the page
>> cache is free.
>> IMO, the condition whether a page cache is free should be 3 in page
>> frame reclaiming. The reason lists as below.
>>
>> When page is allocated, the page->_count is 1(code fragment is code-1 ).
>> And when the page is allocated for reading files from extern disk, the
>> page->_count will increment 1 by page_cache_get() in
>> add_to_page_cache_locked()(code fragment is code-2). When the page is to
>> reclaim, the isolated LRU list also increase the page->_count(code
>> fragment is code-3).
> The page count is initialized to 1, but that does not stay with the
> object.  It's a reference that is passed to the allocating task, which
> drops it again when it's done with the page.  I.e. the pattern is like
> this:
>
> instantiation:
> page = page_cache_alloc()	/* instantiator reference -> 1 */
> add_to_page_cache(page, mapping, offset)
>    get_page(page)		/* page cache reference -> 2 */
> lru_cache_add(page)
>    get_page(page)		/* pagevec reference -> 3 */
> /* ...initiate read, write, associate buffers, ... */
> page_cache_release(page)	/* drop instantiator reference -> 2 + private */
>
> reclaim:
> lru_add_drain()
>    page_cache_release(page)	/* drop pagevec reference -> 1 + private */

IIUC, when add page to lru will lead to add to pagevec firstly, and 
pagevec will take one reference, so if lru will take over the reference 
taken by pagevec when page transmit from pagevec to lru? or just drop 
the reference and lru will not take reference for page?

> __isolate_lru_page(page)
>    page_cache_get(page)		/* reclaim reference -> 2 + private */
> is_page_cache_freeable(page)
> try_to_free_buffers()		/* drop buffer ref -> 2 */
> __remove_mapping()		/* drop page cache and isolator ref -> 0 */
> free_hot_cold_page()
>
> --
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Re: [PATCH] mm: Fixup the condition whether the page cache is free

[Johannes Weiner]

On Wed, Mar 06, 2013 at 09:04:55AM +0800, Simon Jeons wrote:
> Hi Johannes,
> On 03/04/2013 11:09 PM, Johannes Weiner wrote:
> >On Mon, Mar 04, 2013 at 09:54:26AM +0800, Li Haifeng wrote:
> >>When a page cache is to reclaim, we should to decide whether the page
> >>cache is free.
> >>IMO, the condition whether a page cache is free should be 3 in page
> >>frame reclaiming. The reason lists as below.
> >>
> >>When page is allocated, the page->_count is 1(code fragment is code-1 ).
> >>And when the page is allocated for reading files from extern disk, the
> >>page->_count will increment 1 by page_cache_get() in
> >>add_to_page_cache_locked()(code fragment is code-2). When the page is to
> >>reclaim, the isolated LRU list also increase the page->_count(code
> >>fragment is code-3).
> >The page count is initialized to 1, but that does not stay with the
> >object.  It's a reference that is passed to the allocating task, which
> >drops it again when it's done with the page.  I.e. the pattern is like
> >this:
> >
> >instantiation:
> >page = page_cache_alloc()	/* instantiator reference -> 1 */
> >add_to_page_cache(page, mapping, offset)
> >   get_page(page)		/* page cache reference -> 2 */
> >lru_cache_add(page)
> >   get_page(page)		/* pagevec reference -> 3 */
> >/* ...initiate read, write, associate buffers, ... */
> >page_cache_release(page)	/* drop instantiator reference -> 2 + private */
> >
> >reclaim:
> >lru_add_drain()
> >   page_cache_release(page)	/* drop pagevec reference -> 1 + private */
> 
> IIUC, when add page to lru will lead to add to pagevec firstly, and
> pagevec will take one reference, so if lru will take over the
> reference taken by pagevec when page transmit from pagevec to lru?
> or just drop the reference and lru will not take reference for page?

The LRU does not hold a reference, it would not make sense.  The
pagevec only needs one because it would be awkward to remove a
concurrently freed page out of a pagevec, but unlinking a page from
the LRU is easy.  See mm/swap.c::__page_cache_release() and friends.

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Re: [PATCH] mm: Fixup the condition whether the page cache is free

[Simon Jeons]

Hi Johannes,
On 03/07/2013 03:47 AM, Johannes Weiner wrote:
> On Wed, Mar 06, 2013 at 09:04:55AM +0800, Simon Jeons wrote:
>> Hi Johannes,
>> On 03/04/2013 11:09 PM, Johannes Weiner wrote:
>>> On Mon, Mar 04, 2013 at 09:54:26AM +0800, Li Haifeng wrote:
>>>> When a page cache is to reclaim, we should to decide whether the page
>>>> cache is free.
>>>> IMO, the condition whether a page cache is free should be 3 in page
>>>> frame reclaiming. The reason lists as below.
>>>>
>>>> When page is allocated, the page->_count is 1(code fragment is code-1 ).
>>>> And when the page is allocated for reading files from extern disk, the
>>>> page->_count will increment 1 by page_cache_get() in
>>>> add_to_page_cache_locked()(code fragment is code-2). When the page is to
>>>> reclaim, the isolated LRU list also increase the page->_count(code
>>>> fragment is code-3).
>>> The page count is initialized to 1, but that does not stay with the
>>> object.  It's a reference that is passed to the allocating task, which
>>> drops it again when it's done with the page.  I.e. the pattern is like
>>> this:
>>>
>>> instantiation:
>>> page = page_cache_alloc()	/* instantiator reference -> 1 */
>>> add_to_page_cache(page, mapping, offset)
>>>    get_page(page)		/* page cache reference -> 2 */
>>> lru_cache_add(page)
>>>    get_page(page)		/* pagevec reference -> 3 */
>>> /* ...initiate read, write, associate buffers, ... */
>>> page_cache_release(page)	/* drop instantiator reference -> 2 + private */
>>>
>>> reclaim:
>>> lru_add_drain()
>>>    page_cache_release(page)	/* drop pagevec reference -> 1 + private */
>> IIUC, when add page to lru will lead to add to pagevec firstly, and
>> pagevec will take one reference, so if lru will take over the
>> reference taken by pagevec when page transmit from pagevec to lru?
>> or just drop the reference and lru will not take reference for page?
> The LRU does not hold a reference, it would not make sense.  The
> pagevec only needs one because it would be awkward to remove a
> concurrently freed page out of a pagevec, but unlinking a page from
> the LRU is easy.  See mm/swap.c::__page_cache_release() and friends.

Since pagevec is per cpu, when can remove a concurrently freed page out 
of a pagevec happen?


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Re: [PATCH] mm: Fixup the condition whether the page cache is free

[Simon Jeons]

Ping, :-)
On 03/07/2013 09:05 AM, Simon Jeons wrote:
> Hi Johannes,
> On 03/07/2013 03:47 AM, Johannes Weiner wrote:
>> On Wed, Mar 06, 2013 at 09:04:55AM +0800, Simon Jeons wrote:
>>> Hi Johannes,
>>> On 03/04/2013 11:09 PM, Johannes Weiner wrote:
>>>> On Mon, Mar 04, 2013 at 09:54:26AM +0800, Li Haifeng wrote:
>>>>> When a page cache is to reclaim, we should to decide whether the page
>>>>> cache is free.
>>>>> IMO, the condition whether a page cache is free should be 3 in page
>>>>> frame reclaiming. The reason lists as below.
>>>>>
>>>>> When page is allocated, the page->_count is 1(code fragment is 
>>>>> code-1 ).
>>>>> And when the page is allocated for reading files from extern disk, 
>>>>> the
>>>>> page->_count will increment 1 by page_cache_get() in
>>>>> add_to_page_cache_locked()(code fragment is code-2). When the page 
>>>>> is to
>>>>> reclaim, the isolated LRU list also increase the page->_count(code
>>>>> fragment is code-3).
>>>> The page count is initialized to 1, but that does not stay with the
>>>> object.  It's a reference that is passed to the allocating task, which
>>>> drops it again when it's done with the page.  I.e. the pattern is like
>>>> this:
>>>>
>>>> instantiation:
>>>> page = page_cache_alloc()    /* instantiator reference -> 1 */
>>>> add_to_page_cache(page, mapping, offset)
>>>>    get_page(page)        /* page cache reference -> 2 */
>>>> lru_cache_add(page)
>>>>    get_page(page)        /* pagevec reference -> 3 */
>>>> /* ...initiate read, write, associate buffers, ... */
>>>> page_cache_release(page)    /* drop instantiator reference -> 2 + 
>>>> private */
>>>>
>>>> reclaim:
>>>> lru_add_drain()
>>>>    page_cache_release(page)    /* drop pagevec reference -> 1 + 
>>>> private */
>>> IIUC, when add page to lru will lead to add to pagevec firstly, and
>>> pagevec will take one reference, so if lru will take over the
>>> reference taken by pagevec when page transmit from pagevec to lru?
>>> or just drop the reference and lru will not take reference for page?
>> The LRU does not hold a reference, it would not make sense.  The
>> pagevec only needs one because it would be awkward to remove a
>> concurrently freed page out of a pagevec, but unlinking a page from
>> the LRU is easy.  See mm/swap.c::__page_cache_release() and friends.
>
> Since pagevec is per cpu, when can remove a concurrently freed page 
> out of a pagevec happen?
>
>

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Re: [PATCH] mm: Fixup the condition whether the page cache is free

[Johannes Weiner]

On Fri, Mar 08, 2013 at 10:13:25AM +0800, Simon Jeons wrote:
> Ping, :-)
> On 03/07/2013 09:05 AM, Simon Jeons wrote:
> >Hi Johannes,
> >On 03/07/2013 03:47 AM, Johannes Weiner wrote:
> >>On Wed, Mar 06, 2013 at 09:04:55AM +0800, Simon Jeons wrote:
> >>>Hi Johannes,
> >>>On 03/04/2013 11:09 PM, Johannes Weiner wrote:
> >>>>On Mon, Mar 04, 2013 at 09:54:26AM +0800, Li Haifeng wrote:
> >>>>>When a page cache is to reclaim, we should to decide whether the page
> >>>>>cache is free.
> >>>>>IMO, the condition whether a page cache is free should be 3 in page
> >>>>>frame reclaiming. The reason lists as below.
> >>>>>
> >>>>>When page is allocated, the page->_count is 1(code
> >>>>>fragment is code-1 ).
> >>>>>And when the page is allocated for reading files from
> >>>>>extern disk, the
> >>>>>page->_count will increment 1 by page_cache_get() in
> >>>>>add_to_page_cache_locked()(code fragment is code-2). When
> >>>>>the page is to
> >>>>>reclaim, the isolated LRU list also increase the page->_count(code
> >>>>>fragment is code-3).
> >>>>The page count is initialized to 1, but that does not stay with the
> >>>>object.  It's a reference that is passed to the allocating task, which
> >>>>drops it again when it's done with the page.  I.e. the pattern is like
> >>>>this:
> >>>>
> >>>>instantiation:
> >>>>page = page_cache_alloc()    /* instantiator reference -> 1 */
> >>>>add_to_page_cache(page, mapping, offset)
> >>>>   get_page(page)        /* page cache reference -> 2 */
> >>>>lru_cache_add(page)
> >>>>   get_page(page)        /* pagevec reference -> 3 */
> >>>>/* ...initiate read, write, associate buffers, ... */
> >>>>page_cache_release(page)    /* drop instantiator reference
> >>>>-> 2 + private */
> >>>>
> >>>>reclaim:
> >>>>lru_add_drain()
> >>>>   page_cache_release(page)    /* drop pagevec reference ->
> >>>>1 + private */
> >>>IIUC, when add page to lru will lead to add to pagevec firstly, and
> >>>pagevec will take one reference, so if lru will take over the
> >>>reference taken by pagevec when page transmit from pagevec to lru?
> >>>or just drop the reference and lru will not take reference for page?
> >>The LRU does not hold a reference, it would not make sense.  The
> >>pagevec only needs one because it would be awkward to remove a
> >>concurrently freed page out of a pagevec, but unlinking a page from
> >>the LRU is easy.  See mm/swap.c::__page_cache_release() and friends.
> >
> >Since pagevec is per cpu, when can remove a concurrently freed
> >page out of a pagevec happen?

It doesn't because the pagevec holds a reference, as I wrote above.

Feel free to consult the code as well for questions like these ;-)

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Re: [PATCH] mm: Fixup the condition whether the page cache is free

[Simon Jeons]

On 03/08/2013 10:37 AM, Johannes Weiner wrote:
> On Fri, Mar 08, 2013 at 10:13:25AM +0800, Simon Jeons wrote:
>> Ping, :-)
>> On 03/07/2013 09:05 AM, Simon Jeons wrote:
>>> Hi Johannes,
>>> On 03/07/2013 03:47 AM, Johannes Weiner wrote:
>>>> On Wed, Mar 06, 2013 at 09:04:55AM +0800, Simon Jeons wrote:
>>>>> Hi Johannes,
>>>>> On 03/04/2013 11:09 PM, Johannes Weiner wrote:
>>>>>> On Mon, Mar 04, 2013 at 09:54:26AM +0800, Li Haifeng wrote:
>>>>>>> When a page cache is to reclaim, we should to decide whether the page
>>>>>>> cache is free.
>>>>>>> IMO, the condition whether a page cache is free should be 3 in page
>>>>>>> frame reclaiming. The reason lists as below.
>>>>>>>
>>>>>>> When page is allocated, the page->_count is 1(code
>>>>>>> fragment is code-1 ).
>>>>>>> And when the page is allocated for reading files from
>>>>>>> extern disk, the
>>>>>>> page->_count will increment 1 by page_cache_get() in
>>>>>>> add_to_page_cache_locked()(code fragment is code-2). When
>>>>>>> the page is to
>>>>>>> reclaim, the isolated LRU list also increase the page->_count(code
>>>>>>> fragment is code-3).
>>>>>> The page count is initialized to 1, but that does not stay with the
>>>>>> object.  It's a reference that is passed to the allocating task, which
>>>>>> drops it again when it's done with the page.  I.e. the pattern is like
>>>>>> this:
>>>>>>
>>>>>> instantiation:
>>>>>> page = page_cache_alloc()    /* instantiator reference -> 1 */
>>>>>> add_to_page_cache(page, mapping, offset)
>>>>>>    get_page(page)        /* page cache reference -> 2 */
>>>>>> lru_cache_add(page)
>>>>>>    get_page(page)        /* pagevec reference -> 3 */
>>>>>> /* ...initiate read, write, associate buffers, ... */
>>>>>> page_cache_release(page)    /* drop instantiator reference
>>>>>> -> 2 + private */
>>>>>>
>>>>>> reclaim:
>>>>>> lru_add_drain()
>>>>>>    page_cache_release(page)    /* drop pagevec reference ->
>>>>>> 1 + private */
>>>>> IIUC, when add page to lru will lead to add to pagevec firstly, and
>>>>> pagevec will take one reference, so if lru will take over the
>>>>> reference taken by pagevec when page transmit from pagevec to lru?
>>>>> or just drop the reference and lru will not take reference for page?
>>>> The LRU does not hold a reference, it would not make sense.  The
>>>> pagevec only needs one because it would be awkward to remove a
>>>> concurrently freed page out of a pagevec, but unlinking a page from
>>>> the LRU is easy.  See mm/swap.c::__page_cache_release() and friends.
>>> Since pagevec is per cpu, when can remove a concurrently freed
>>> page out of a pagevec happen?
> It doesn't because the pagevec holds a reference, as I wrote above.

I mean since pagevec is per cpu, how can remove a concurrently freed 
page out of a pagevec happen? If it doesn't happen pagevec don't need to 
hold a reference. :-)

>
> Feel free to consult the code as well for questions like these ;-)

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Re: [PATCH] mm: Fixup the condition whether the page cache is free

[Johannes Weiner]

On Fri, Mar 08, 2013 at 10:48:31AM +0800, Simon Jeons wrote:
> On 03/08/2013 10:37 AM, Johannes Weiner wrote:
> >On Fri, Mar 08, 2013 at 10:13:25AM +0800, Simon Jeons wrote:
> >>Ping, :-)
> >>On 03/07/2013 09:05 AM, Simon Jeons wrote:
> >>>Hi Johannes,
> >>>On 03/07/2013 03:47 AM, Johannes Weiner wrote:
> >>>>On Wed, Mar 06, 2013 at 09:04:55AM +0800, Simon Jeons wrote:
> >>>>>Hi Johannes,
> >>>>>On 03/04/2013 11:09 PM, Johannes Weiner wrote:
> >>>>>>On Mon, Mar 04, 2013 at 09:54:26AM +0800, Li Haifeng wrote:
> >>>>>>>When a page cache is to reclaim, we should to decide whether the page
> >>>>>>>cache is free.
> >>>>>>>IMO, the condition whether a page cache is free should be 3 in page
> >>>>>>>frame reclaiming. The reason lists as below.
> >>>>>>>
> >>>>>>>When page is allocated, the page->_count is 1(code
> >>>>>>>fragment is code-1 ).
> >>>>>>>And when the page is allocated for reading files from
> >>>>>>>extern disk, the
> >>>>>>>page->_count will increment 1 by page_cache_get() in
> >>>>>>>add_to_page_cache_locked()(code fragment is code-2). When
> >>>>>>>the page is to
> >>>>>>>reclaim, the isolated LRU list also increase the page->_count(code
> >>>>>>>fragment is code-3).
> >>>>>>The page count is initialized to 1, but that does not stay with the
> >>>>>>object.  It's a reference that is passed to the allocating task, which
> >>>>>>drops it again when it's done with the page.  I.e. the pattern is like
> >>>>>>this:
> >>>>>>
> >>>>>>instantiation:
> >>>>>>page = page_cache_alloc()    /* instantiator reference -> 1 */
> >>>>>>add_to_page_cache(page, mapping, offset)
> >>>>>>   get_page(page)        /* page cache reference -> 2 */
> >>>>>>lru_cache_add(page)
> >>>>>>   get_page(page)        /* pagevec reference -> 3 */
> >>>>>>/* ...initiate read, write, associate buffers, ... */
> >>>>>>page_cache_release(page)    /* drop instantiator reference
> >>>>>>-> 2 + private */
> >>>>>>
> >>>>>>reclaim:
> >>>>>>lru_add_drain()
> >>>>>>   page_cache_release(page)    /* drop pagevec reference ->
> >>>>>>1 + private */
> >>>>>IIUC, when add page to lru will lead to add to pagevec firstly, and
> >>>>>pagevec will take one reference, so if lru will take over the
> >>>>>reference taken by pagevec when page transmit from pagevec to lru?
> >>>>>or just drop the reference and lru will not take reference for page?
> >>>>The LRU does not hold a reference, it would not make sense.  The
> >>>>pagevec only needs one because it would be awkward to remove a
> >>>>concurrently freed page out of a pagevec, but unlinking a page from
> >>>>the LRU is easy.  See mm/swap.c::__page_cache_release() and friends.
> >>>Since pagevec is per cpu, when can remove a concurrently freed
> >>>page out of a pagevec happen?
> >It doesn't because the pagevec holds a reference, as I wrote above.
> 
> I mean since pagevec is per cpu, how can remove a concurrently freed
> page out of a pagevec happen? If it doesn't happen pagevec don't
> need to hold a reference. :-)

It has nothing to do with the pagevec being per CPU.  The page may get
truncated or reclaimed and have every other reference being dropped
while it sits on the pagevec.

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Re: [PATCH] mm: Fixup the condition whether the page cache is free

[Simon Jeons]

Hi Hugh and Johannes,
On 03/08/2013 11:16 AM, Johannes Weiner wrote:
> On Fri, Mar 08, 2013 at 10:48:31AM +0800, Simon Jeons wrote:
>> On 03/08/2013 10:37 AM, Johannes Weiner wrote:
>>> On Fri, Mar 08, 2013 at 10:13:25AM +0800, Simon Jeons wrote:
>>>> Ping, :-)
>>>> On 03/07/2013 09:05 AM, Simon Jeons wrote:
>>>>> Hi Johannes,
>>>>> On 03/07/2013 03:47 AM, Johannes Weiner wrote:
>>>>>> On Wed, Mar 06, 2013 at 09:04:55AM +0800, Simon Jeons wrote:
>>>>>>> Hi Johannes,
>>>>>>> On 03/04/2013 11:09 PM, Johannes Weiner wrote:
>>>>>>>> On Mon, Mar 04, 2013 at 09:54:26AM +0800, Li Haifeng wrote:
>>>>>>>>> When a page cache is to reclaim, we should to decide whether the page
>>>>>>>>> cache is free.
>>>>>>>>> IMO, the condition whether a page cache is free should be 3 in page
>>>>>>>>> frame reclaiming. The reason lists as below.
>>>>>>>>>
>>>>>>>>> When page is allocated, the page->_count is 1(code
>>>>>>>>> fragment is code-1 ).
>>>>>>>>> And when the page is allocated for reading files from
>>>>>>>>> extern disk, the
>>>>>>>>> page->_count will increment 1 by page_cache_get() in
>>>>>>>>> add_to_page_cache_locked()(code fragment is code-2). When
>>>>>>>>> the page is to
>>>>>>>>> reclaim, the isolated LRU list also increase the page->_count(code
>>>>>>>>> fragment is code-3).
>>>>>>>> The page count is initialized to 1, but that does not stay with the
>>>>>>>> object.  It's a reference that is passed to the allocating task, which
>>>>>>>> drops it again when it's done with the page.  I.e. the pattern is like
>>>>>>>> this:
>>>>>>>>
>>>>>>>> instantiation:
>>>>>>>> page = page_cache_alloc()    /* instantiator reference -> 1 */
>>>>>>>> add_to_page_cache(page, mapping, offset)
>>>>>>>>    get_page(page)        /* page cache reference -> 2 */
>>>>>>>> lru_cache_add(page)
>>>>>>>>    get_page(page)        /* pagevec reference -> 3 */
>>>>>>>> /* ...initiate read, write, associate buffers, ... */
>>>>>>>> page_cache_release(page)    /* drop instantiator reference
>>>>>>>> -> 2 + private */
>>>>>>>>
>>>>>>>> reclaim:
>>>>>>>> lru_add_drain()
>>>>>>>>    page_cache_release(page)    /* drop pagevec reference ->
>>>>>>>> 1 + private */
>>>>>>> IIUC, when add page to lru will lead to add to pagevec firstly, and
>>>>>>> pagevec will take one reference, so if lru will take over the
>>>>>>> reference taken by pagevec when page transmit from pagevec to lru?
>>>>>>> or just drop the reference and lru will not take reference for page?
>>>>>> The LRU does not hold a reference, it would not make sense.  The
>>>>>> pagevec only needs one because it would be awkward to remove a
>>>>>> concurrently freed page out of a pagevec, but unlinking a page from
>>>>>> the LRU is easy.  See mm/swap.c::__page_cache_release() and friends.
>>>>> Since pagevec is per cpu, when can remove a concurrently freed
>>>>> page out of a pagevec happen?
>>> It doesn't because the pagevec holds a reference, as I wrote above.
>> I mean since pagevec is per cpu, how can remove a concurrently freed
>> page out of a pagevec happen? If it doesn't happen pagevec don't
>> need to hold a reference. :-)
> It has nothing to do with the pagevec being per CPU.  The page may get
> truncated or reclaimed and have every other reference being dropped
> while it sits on the pagevec.

In function shmem_replace_page, there are twice call of 
page_cache_release for oldpage, one is for pre_new_page, the other is 
for page cache, but if page is still in pagevec,  pagevec has one 
reference and oldpage can't be freed, is it a bug?

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