On Thu, Sep 11, 2014 at 04:53:52PM -0400, Dan Streetman wrote:
> When zsmalloc creates a new zspage, it initializes each object it contains
> with a link to the next object, so that the zspage has a singly-linked list
> of its free objects. However, the logic that sets up the links is wrong,
> and in the case of objects that are precisely aligned with the page boundries
> (e.g. a zspage with objects that are 1/2 PAGE_SIZE) the first object on the
> next page is skipped, due to incrementing the offset twice. The logic can be
> simplified, as it doesn't need to calculate how many objects can fit on the
> current page; simply checking the offset for each object is enough.
If objects are precisely aligned with the page boundary, pages_per_zspage
should be 1 so there is no next page.
>
> Change zsmalloc init_zspage() logic to iterate through each object on
> each of its pages, checking the offset to verify the object is on the
> current page before linking it into the zspage.
>
> Signed-off-by: Dan Streetman <ddstreet@xxxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> ---
> mm/zsmalloc.c | 14 +++++---------
> 1 file changed, 5 insertions(+), 9 deletions(-)
>
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index c4a9157..03aa72f 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -628,7 +628,7 @@ static void init_zspage(struct page *first_page, struct size_class *class)
> while (page) {
> struct page *next_page;
> struct link_free *link;
> - unsigned int i, objs_on_page;
> + unsigned int i = 1;
>
> /*
> * page->index stores offset of first object starting
> @@ -641,14 +641,10 @@ static void init_zspage(struct page *first_page, struct size_class *class)
>
> link = (struct link_free *)kmap_atomic(page) +
> off / sizeof(*link);
> - objs_on_page = (PAGE_SIZE - off) / class->size;
>
> - for (i = 1; i <= objs_on_page; i++) {
> - off += class->size;
> - if (off < PAGE_SIZE) {
> - link->next = obj_location_to_handle(page, i);
> - link += class->size / sizeof(*link);
> - }
> + while ((off += class->size) < PAGE_SIZE) {
> + link->next = obj_location_to_handle(page, i++);
> + link += class->size / sizeof(*link);
> }
>
> /*
> @@ -660,7 +656,7 @@ static void init_zspage(struct page *first_page, struct size_class *class)
> link->next = obj_location_to_handle(next_page, 0);
> kunmap_atomic(link);
> page = next_page;
> - off = (off + class->size) % PAGE_SIZE;
> + off %= PAGE_SIZE;
> }
> }
>
> --
> 1.8.3.1
>
> --
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--
Kind regards,
Minchan Kim
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