Re: [PATCH v4 02/22] mm/zsmalloc: use zpdesc in trylock_zspage/lock_zspage

[Alex Shi]

On 8/3/24 3:02 AM, Vishal Moola wrote:
> On Mon, Jul 29, 2024 at 07:25:14PM +0800, alexs@xxxxxxxxxx wrote:
>> From: Alex Shi <alexs@xxxxxxxxxx>
>>
>> To use zpdesc in trylock_zspage/lock_zspage funcs, we add couple of helpers:
>> zpdesc_lock/zpdesc_unlock/zpdesc_trylock/zpdesc_wait_locked and
>> zpdesc_get/zpdesc_put for this purpose.
> 
> You should always include the "()" following function names. It just
> makes everything more readable.

Thanks for reminder, I will update the commit log.

> 
>> Here we use the folio series func in guts for 2 reasons, one zswap.zpool
>> only get single page, and use folio could save some compound_head checking;
>> two, folio_put could bypass devmap checking that we don't need.
>>
>> Originally-by: Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx>
>> Signed-off-by: Alex Shi <alexs@xxxxxxxxxx>
>> ---
>>  mm/zpdesc.h   | 30 ++++++++++++++++++++++++
>>  mm/zsmalloc.c | 64 ++++++++++++++++++++++++++++++++++-----------------
>>  2 files changed, 73 insertions(+), 21 deletions(-)
>>
>> diff --git a/mm/zpdesc.h b/mm/zpdesc.h
>> index 2dbef231f616..3b04197cec9d 100644
>> --- a/mm/zpdesc.h
>> +++ b/mm/zpdesc.h
>> @@ -63,4 +63,34 @@ static_assert(sizeof(struct zpdesc) <= sizeof(struct page));
>>  	const struct page *:		(const struct zpdesc *)(p),	\
>>  	struct page *:			(struct zpdesc *)(p)))
>>  
>> +static inline void zpdesc_lock(struct zpdesc *zpdesc)
>> +{
>> +	folio_lock(zpdesc_folio(zpdesc));
>> +}
>> +
>> +static inline bool zpdesc_trylock(struct zpdesc *zpdesc)
>> +{
>> +	return folio_trylock(zpdesc_folio(zpdesc));
>> +}
>> +
>> +static inline void zpdesc_unlock(struct zpdesc *zpdesc)
>> +{
>> +	folio_unlock(zpdesc_folio(zpdesc));
>> +}
>> +
>> +static inline void zpdesc_wait_locked(struct zpdesc *zpdesc)
>> +{
>> +	folio_wait_locked(zpdesc_folio(zpdesc));
>> +}
> 
> The more I look at zsmalloc, the more skeptical I get about it "needing"
> the folio_lock. At a glance it seems like a zspage already has its own lock,
> and the migration doesn't appear to be truly physical? There's probably
> something I'm missing... it would make this code a lot simpler to drop
> many of the folio locks.

folio series could save about 6.3% object code... Anyway I don't insist on
it. Just want a double confirm, could we keep the code size saving? :)

> 
>> +
>> +static inline void zpdesc_get(struct zpdesc *zpdesc)
>> +{
>> +	folio_get(zpdesc_folio(zpdesc));
>> +}
>> +
>> +static inline void zpdesc_put(struct zpdesc *zpdesc)
>> +{
>> +	folio_put(zpdesc_folio(zpdesc));
>> +}
>> +
>>  #endif
>> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
>> index a532851025f9..243677a9c6d2 100644
>> --- a/mm/zsmalloc.c
>> +++ b/mm/zsmalloc.c
>> @@ -433,13 +433,17 @@ static __maybe_unused int is_first_page(struct page *page)
>>  	return PagePrivate(page);
>>  }
>>  
>> +static int is_first_zpdesc(struct zpdesc *zpdesc)
>> +{
>> +	return PagePrivate(zpdesc_page(zpdesc));
>> +}
>> +
> 
> I feel like we might not even need to use the PG_private flag for
> zpages? It seems to me like its just used for sanity checking. Can
> zpage->first_page ever not point to the first zpdesc?

Yes, the PG_private is only for sanity checking now. But zspage.first_zpdesc
are still used widely and must point to the first subpage. 
I believe we could safely remove this page flag, maybe next patchset?

> 
> For the purpose of introducing the memdesc its fine to continue using
> it; just some food for thought.

Yes.
 
Thanks a lot! :)