On Mon 09-12-19 14:53:42, 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/ > > Suggested-by: Jan Kara <jack@xxxxxxx> > Suggested-by: Jérôme Glisse <jglisse@xxxxxxxxxx> > Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx> Looks nice, some comments below... > +/* > + * 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 with to use FOLL_PIN > + * or FOLL_GET behavior, when incrementing the page's refcount. > + */ > +static 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); > +} > + > +/** > + * 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 with to use FOLL_PIN ^^^ whether > + * 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. > + */ > +void grab_page(struct page *page, unsigned int flags) > +{ > + if (flags & FOLL_GET) > + get_page(page); > + else if (flags & FOLL_PIN) { > + get_page(page); > + WARN_ON_ONCE(flags & FOLL_GET); > + /* > + * Use get_page(), above, to do the refcount error > + * checking. Then just add in the remaining references: > + */ > + page_ref_add(page, GUP_PIN_COUNTING_BIAS - 1); This is wrong for two reasons: 1) You miss compound_head() indirection from get_page() for this page_ref_add(). 2) page_ref_add() could overflow the counter without noticing. Especially with GUP_PIN_COUNTING_BIAS being non-trivial, it is realistic that an attacker might try to overflow the page refcount and we have to protect the kernel against that. So I think that all the places that would use grab_page() actually need to use try_grab_page() and then gracefully deal with the failure. > @@ -278,11 +425,23 @@ static struct page *follow_page_pte(struct vm_area_struct *vma, > goto retry; > } > > - if (flags & FOLL_GET) { > + if (flags & (FOLL_PIN | FOLL_GET)) { > + /* > + * Allow try_get_page() to take care of error handling, for > + * both cases: FOLL_GET or FOLL_PIN: > + */ > if (unlikely(!try_get_page(page))) { > page = ERR_PTR(-ENOMEM); > goto out; > } > + > + if (flags & FOLL_PIN) { > + WARN_ON_ONCE(flags & FOLL_GET); > + > + /* We got a +1 refcount from try_get_page(), above. */ > + page_ref_add(page, GUP_PIN_COUNTING_BIAS - 1); > + __update_proc_vmstat(page, NR_FOLL_PIN_REQUESTED, 1); > + } > } The same problem here as above, plus this place should use the same try_grab..() helper, shouldn't it? > @@ -544,8 +703,8 @@ static struct page *follow_page_mask(struct vm_area_struct *vma, > /* make this handle hugepd */ > page = follow_huge_addr(mm, address, flags & FOLL_WRITE); > if (!IS_ERR(page)) { > - BUG_ON(flags & FOLL_GET); > - return page; > + WARN_ON_ONCE(flags & (FOLL_GET | FOLL_PIN)); > + return NULL; I agree with the change to WARN_ON_ONCE but why is correct the change of the return value? Note that this is actually a "success branch". Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
