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    Subject[RFC v2 04/10] mm: remove set_page_count() from page_frag_alloc_align
    Date
    set_page_count() unconditionally resets the value of _ref_count and that
    is dangerous, as it is not programmatically verified. Instead we rely on
    comments like: "OK, page count is 0, we can safely set it".

    Add a new refcount function: page_ref_add_return() to return the new
    refcount value after adding to it. Use the return value to verify that
    the _ref_count was indeed the expected one.

    Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
    ---
    include/linux/page_ref.h | 11 +++++++++++
    mm/page_alloc.c | 6 ++++--
    2 files changed, 15 insertions(+), 2 deletions(-)

    diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h
    index f3c61dc6344a..27880aca2e2f 100644
    --- a/include/linux/page_ref.h
    +++ b/include/linux/page_ref.h
    @@ -115,6 +115,17 @@ static inline void init_page_count(struct page *page)
    set_page_count(page, 1);
    }

    +static inline int page_ref_add_return(struct page *page, int nr)
    +{
    + int old_val = atomic_fetch_add(nr, &page->_refcount);
    + int new_val = old_val + nr;
    +
    + VM_BUG_ON_PAGE((unsigned int)new_val < (unsigned int)old_val, page);
    + if (page_ref_tracepoint_active(page_ref_mod_and_return))
    + __page_ref_mod_and_return(page, nr, new_val);
    + return new_val;
    +}
    +
    static inline void page_ref_add(struct page *page, int nr)
    {
    int old_val = atomic_fetch_add(nr, &page->_refcount);
    diff --git a/mm/page_alloc.c b/mm/page_alloc.c
    index c5952749ad40..e8e88111028a 100644
    --- a/mm/page_alloc.c
    +++ b/mm/page_alloc.c
    @@ -5516,6 +5516,7 @@ void *page_frag_alloc_align(struct page_frag_cache *nc,
    unsigned int size = PAGE_SIZE;
    struct page *page;
    int offset;
    + int refcnt;

    if (unlikely(!nc->va)) {
    refill:
    @@ -5554,8 +5555,9 @@ void *page_frag_alloc_align(struct page_frag_cache *nc,
    /* if size can vary use size else just use PAGE_SIZE */
    size = nc->size;
    #endif
    - /* OK, page count is 0, we can safely set it */
    - set_page_count(page, PAGE_FRAG_CACHE_MAX_SIZE + 1);
    + /* page count is 0, set it to PAGE_FRAG_CACHE_MAX_SIZE + 1 */
    + refcnt = page_ref_add_return(page, PAGE_FRAG_CACHE_MAX_SIZE + 1);
    + VM_BUG_ON_PAGE(refcnt != PAGE_FRAG_CACHE_MAX_SIZE + 1, page);

    /* reset page count bias and offset to start of new frag */
    nc->pagecnt_bias = PAGE_FRAG_CACHE_MAX_SIZE + 1;
    --
    2.34.0.rc1.387.gb447b232ab-goog
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