On 12/11/19 3:28 AM, Jan Kara wrote: ...
The patch looks mostly good to me now. Just a few smaller comments below.
Suggested-by: Jan Kara jack@suse.cz Suggested-by: Jérôme Glisse jglisse@redhat.com Reviewed-by: Jan Kara jack@suse.cz Reviewed-by: Jérôme Glisse jglisse@redhat.com Reviewed-by: Ira Weiny ira.weiny@intel.com
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.
OK, I've removed those reviewed-by's. (I felt bad about dropping them, after people had devoted time to reviewing, but I do see that it's wrong to imply that they've reviewed this much much larger thing.)
...
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...
Done. It is only a modest code-level change, at least the way I've done it, which is setting FOLL_GET for try_grab_compound_head(). In order to do that, I set it at the top of the internal gup fast calling stacks, which is actually a good design anyway: gup fast is logically doing FOLL_GET in all cases. So setting the flag internally is accurate and consistent with the overall design.
...
@@ -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.
OK, I've moved it up, adding a "subpage" variable in order to make that work.
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
Yes. I've removed it, good spot.
thanks,