On Tue 10-12-19 18:53:16, John Hubbard wrote: > Add tracking of pages that were pinned via FOLL_PIN. > > As mentioned in the FOLL_PIN documentation, callers who effectively set > FOLL_PIN are required to ultimately free such pages via unpin_user_page(). > The effect is similar to FOLL_GET, and may be thought of as "FOLL_GET > for DIO and/or RDMA use". > > Pages that have been pinned via FOLL_PIN are identifiable via a > new function call: > > bool page_dma_pinned(struct page *page); > > What to do in response to encountering such a page, is left to later > patchsets. There is discussion about this in [1], [2], and [3]. > > This also changes a BUG_ON(), to a WARN_ON(), in follow_page_mask(). > > [1] Some slow progress on get_user_pages() (Apr 2, 2019): > https://lwn.net/Articles/784574/ > [2] DMA and get_user_pages() (LPC: Dec 12, 2018): > https://lwn.net/Articles/774411/ > [3] The trouble with get_user_pages() (Apr 30, 2018): > https://lwn.net/Articles/753027/ The patch looks mostly good to me now. Just a few smaller comments below. > Suggested-by: Jan Kara <jack@xxxxxxx> > Suggested-by: Jérôme Glisse <jglisse@xxxxxxxxxx> > Reviewed-by: Jan Kara <jack@xxxxxxx> > Reviewed-by: Jérôme Glisse <jglisse@xxxxxxxxxx> > Reviewed-by: Ira Weiny <ira.weiny@xxxxxxxxx> I think you inherited here the Reviewed-by tags from the "add flags" patch you've merged into this one but that's not really fair since this patch does much more... In particular I didn't give my Reviewed-by tag for this patch yet. > +/* > + * try_grab_compound_head() - attempt to elevate a page's refcount, by a > + * flags-dependent amount. > + * > + * This has a default assumption of "use FOLL_GET behavior, if FOLL_PIN is not > + * set". > + * > + * "grab" names in this file mean, "look at flags to decide whether to use > + * FOLL_PIN or FOLL_GET behavior, when incrementing the page's refcount. > + */ > +static __maybe_unused struct page *try_grab_compound_head(struct page *page, > + int refs, > + unsigned int flags) > +{ > + if (flags & FOLL_PIN) > + return try_pin_compound_head(page, refs); > + > + return try_get_compound_head(page, refs); > +} I somewhat wonder about the asymmetry of try_grab_compound_head() vs try_grab_page() in the treatment of 'flags'. How costly would it be to make them symmetric (i.e., either set FOLL_GET for try_grab_compound_head() callers or make sure one of FOLL_GET, FOLL_PIN is set for try_grab_page())? Because this difference looks like a subtle catch in the long run... > + > +/** > + * try_grab_page() - elevate a page's refcount by a flag-dependent amount > + * > + * This might not do anything at all, depending on the flags argument. > + * > + * "grab" names in this file mean, "look at flags to decide whether to use > + * FOLL_PIN or FOLL_GET behavior, when incrementing the page's refcount. > + * > + * @page: pointer to page to be grabbed > + * @flags: gup flags: these are the FOLL_* flag values. > + * > + * Either FOLL_PIN or FOLL_GET (or neither) may be set, but not both at the same > + * time. (That's true throughout the get_user_pages*() and pin_user_pages*() > + * APIs.) Cases: > + * > + * FOLL_GET: page's refcount will be incremented by 1. > + * FOLL_PIN: page's refcount will be incremented by GUP_PIN_COUNTING_BIAS. > + * > + * Return: true for success, or if no action was required (if neither FOLL_PIN > + * nor FOLL_GET was set, nothing is done). False for failure: FOLL_GET or > + * FOLL_PIN was set, but the page could not be grabbed. > + */ > +bool __must_check try_grab_page(struct page *page, unsigned int flags) > +{ > + if (flags & FOLL_GET) > + return try_get_page(page); > + else if (flags & FOLL_PIN) { > + page = compound_head(page); > + WARN_ON_ONCE(flags & FOLL_GET); > + > + if (WARN_ON_ONCE(page_ref_zero_or_close_to_bias_overflow(page))) > + return false; > + > + page_ref_add(page, GUP_PIN_COUNTING_BIAS); > + __update_proc_vmstat(page, NR_FOLL_PIN_REQUESTED, 1); > + } > + > + return true; > +} ... > @@ -1522,8 +1536,8 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma, > skip_mlock: > page += (addr & ~HPAGE_PMD_MASK) >> PAGE_SHIFT; > VM_BUG_ON_PAGE(!PageCompound(page) && !is_zone_device_page(page), page); > - if (flags & FOLL_GET) > - get_page(page); > + if (!try_grab_page(page, flags)) > + page = ERR_PTR(-EFAULT); I think you need to also move the try_grab_page() earlier in the function. At this point the page may be marked as mlocked and you'd need to undo that in case try_grab_page() fails. > diff --git a/mm/hugetlb.c b/mm/hugetlb.c > index ac65bb5e38ac..0aab6fe0072f 100644 > --- a/mm/hugetlb.c > +++ b/mm/hugetlb.c > @@ -4356,7 +4356,13 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma, > same_page: > if (pages) { > pages[i] = mem_map_offset(page, pfn_offset); > - get_page(pages[i]); > + if (!try_grab_page(pages[i], flags)) { > + spin_unlock(ptl); > + remainder = 0; > + err = -ENOMEM; > + WARN_ON_ONCE(1); > + break; > + } > } This function does a refcount overflow check early so that it doesn't have to do try_get_page() here. So that check can be now removed when you do try_grab_page() here anyway since that early check seems to be just a tiny optimization AFAICT. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
