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

Alex Shi
2024-08-05T07:55:33Z

On 8/3/24 3:02 AM, Vishal Moola wrote:
> On Mon, Jul 29, 2024 at 07:25:14PM +0800, alexs@kernel.org wrote:
>> From: Alex Shi <alexs@kernel.org>
>>
>> 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@gmail.com>
>> Signed-off-by: Alex Shi <alexs@kernel.org>
>> ---
>> 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! :)