Messages in this thread Patch in this message | | | Date | Tue, 23 Sep 2014 13:45:12 +0900 | From | Minchan Kim <> | Subject | Re: [PATCH v1 1/5] zram: generalize swap_slot_free_notify |
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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@kernel.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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