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Date: Tue, 23 Sep 2014 13:45:12 +0900
From: Minchan Kim <minchan@...nel.org>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
Hugh Dickins <hughd@...gle.com>, Shaohua Li <shli@...nel.org>,
Jerome Marchand <jmarchan@...hat.com>,
Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
Dan Streetman <ddstreet@...e.org>,
Nitin Gupta <ngupta@...are.org>,
Luigi Semenzato <semenzato@...gle.com>, juno.choi@....com
Subject: Re: [PATCH v1 1/5] zram: generalize swap_slot_free_notify
Hi Andrew,
On Mon, Sep 22, 2014 at 01:41:09PM -0700, Andrew Morton wrote:
> On Mon, 22 Sep 2014 09:03:07 +0900 Minchan Kim <minchan@...nel.org> wrote:
>
> > Currently, swap_slot_free_notify is used for zram to free
> > duplicated copy page for memory efficiency when it knows
> > there is no reference to the swap slot.
> >
> > This patch generalizes it to be able to use for other
> > swap hint to communicate with VM.
> >
>
> I really think we need to do a better job of documenting the code.
>
> > index 94d93b1f8b53..c262bfbeafa9 100644
> > --- a/Documentation/filesystems/Locking
> > +++ b/Documentation/filesystems/Locking
> > @@ -405,7 +405,7 @@ prototypes:
> > void (*unlock_native_capacity) (struct gendisk *);
> > int (*revalidate_disk) (struct gendisk *);
> > int (*getgeo)(struct block_device *, struct hd_geometry *);
> > - void (*swap_slot_free_notify) (struct block_device *, unsigned long);
> > + int (*swap_hint) (struct block_device *, unsigned int, void *);
> >
> > locking rules:
> > bd_mutex
> > @@ -418,7 +418,7 @@ media_changed: no
> > unlock_native_capacity: no
> > revalidate_disk: no
> > getgeo: no
> > -swap_slot_free_notify: no (see below)
> > +swap_hint: no (see below)
>
> This didn't tell anyone anythnig much.
Yeb. :(
>
> > index d78b245bae06..22a37764c409 100644
> > --- a/drivers/block/zram/zram_drv.c
> > +++ b/drivers/block/zram/zram_drv.c
> > @@ -926,7 +926,8 @@ error:
> > bio_io_error(bio);
> > }
> >
> > -static void zram_slot_free_notify(struct block_device *bdev,
> > +/* this callback is with swap_lock and sometimes page table lock held */
>
> OK, that was useful.
>
> It's called "page_table_lock".
>
> Also *which* page_table_lock? current->mm?
It depends on ALLOC_SPLIT_PTLOCKS so it could be page->ptl, too.
So, it would be better to call it as *ptlock*?
Since it's ptlock, it isn't related to which mm struct.
What we should make sure is just ptlock which belong to the page
table pointed to this swap page.
So, I want this.
diff --git a/Documentation/filesystems/Locking b/Documentation/filesystems/Locking
index c262bfbeafa9..19d2726e34f4 100644
--- a/Documentation/filesystems/Locking
+++ b/Documentation/filesystems/Locking
@@ -423,8 +423,8 @@ swap_hint: no (see below)
media_changed, unlock_native_capacity and revalidate_disk are called only from
check_disk_change().
-swap_slot_free_notify is called with swap_lock and sometimes the page lock
-held.
+swap_hint is called with swap_info_struct->lock and sometimes the ptlock
+of the page table pointed to the swap page.
--------------------------- file_operations -------------------------------
>
> > +static int zram_slot_free_notify(struct block_device *bdev,
> > unsigned long index)
> > {
> > struct zram *zram;
> >
> > ...
> >
> > --- a/include/linux/blkdev.h
> > +++ b/include/linux/blkdev.h
> > @@ -1609,6 +1609,10 @@ static inline bool blk_integrity_is_initialized(struct gendisk *g)
> >
> > #endif /* CONFIG_BLK_DEV_INTEGRITY */
> >
> > +enum swap_blk_hint {
> > + SWAP_FREE,
> > +};
>
> This would be a great place to document SWAP_FREE.
Yes,
>
> > struct block_device_operations {
> > int (*open) (struct block_device *, fmode_t);
> > void (*release) (struct gendisk *, fmode_t);
> > @@ -1624,8 +1628,7 @@ struct block_device_operations {
> > void (*unlock_native_capacity) (struct gendisk *);
> > int (*revalidate_disk) (struct gendisk *);
> > int (*getgeo)(struct block_device *, struct hd_geometry *);
> > - /* this callback is with swap_lock and sometimes page table lock held */
> > - void (*swap_slot_free_notify) (struct block_device *, unsigned long);
> > + int (*swap_hint)(struct block_device *, unsigned int, void *);
>
> And this would be a suitable place to document ->swap_hint().
If we consider to be able to add more hints in future so it could
be verbose, IMO, it would be better to desribe it in enum swap_hint. :)
>
> - Hint from who to who? Is it the caller providing the callee a hint
> or is the caller asking the callee for a hint?
>
> - What is the meaning of the return value?
>
> - What are the meaning of the arguments?
Okay.
>
> Please don't omit the argument names like this. They are useful! How
> is a reader to know what that "unsigned int" and "void *" actually
> *do*?
Yes.
>
> The second arg-which-doesn't-have-a-name should have had type
> swap_blk_hint, yes?
Yes.
>
> swap_blk_hint should be called swap_block_hint. I assume that's what
> "blk" means. Why does the name have "block" in there anyway? It has
> something to do with disk blocks? How is anyone supposed to work that
> out?
Yeb, I think we don't need block in name. I will remove it.
>
> ->swap_hint was converted to return an `int', but all the callers
> simply ignore the return value.
You're right. All caller doesn't use it in this patch but this patch
makes it generalize and in later patch, SWAP_FULL will use it so
I want to make return as int. One more thought, SWAP_FULL could use
void * as output param so we could remove return value, finally.
I will consider it, too.
>
> --
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--
Kind regards,
Minchan Kim
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