Re: [PATCH v3 2/2] mm,thp: Add experimental config option RO_EXEC_FILEMAP_HUGE_FAULT_THP

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Mon, Aug 05, 2019 at 09:56:45AM -0600, William Kucharski wrote:
> >> I don't really care if the start of the VMA is suitable, just whether I can map
> >> the current faulting page with a THP. As far as I know, there's nothing wrong
> >> with mapping all the pages before the VMA hits a properly aligned bound with
> >> PAGESIZE pages and then aligned chunks in the middle with THP.
> > 
> > You cannot map any paged as huge into wrongly aligned VMA.
> > 
> > THP's ->index must be aligned to HPAGE_PMD_NR, so if the combination VMA's
> > ->vm_start and ->vm_pgoff doesn't allow for this, you must fallback to
> > mapping the page with PTEs. I don't see it handled properly here.
> 
> It was my assumption that if say a VMA started at an address say one page
> before a large page alignment, you could map that page with a PAGESIZE
> page but if VMA size allowed, there was a fault on the next page, and
> VMA size allowed, you could map that next range with a large page, taking
> taking the approach of mapping chunks of the VMA with the largest page
> possible.
> 
> Is it that the start of the VMA must always align or that the entire VMA
> must be properly aligned and a multiple of the PMD size (so you either map
> with all large pages or none)?

IIUC, you are missing ->vm_pgoff from the picture. The newly allocated
page must land into page cache aligned on HPAGE_PMD_NR boundary. In other
word you cannout have huge page with ->index, let say, 1.

VMA is only suitable for at least one file-THP page if:

 - (vma->vm_start >> PAGE_SHIFT) % (HPAGE_PMD_NR - 1) is equal to
    vma->vm_pgoff % (HPAGE_PMD_NR - 1)

    This guarantees right alignment in the backing page cache.

 - *and* vma->vm_end - round_up(vma->vm_start, HPAGE_PMD_SIZE) is equal or
   greater than HPAGE_PMD_SIZE.

Does it make sense?

> 
> >> This is the page that content was just read to; readpage() will unlock the page
> >> when it is done with I/O, but the page needs to be locked before it's inserted
> >> into the page cache.
> > 
> > Then you must to lock the page properly with lock_page().
> > 
> > __SetPageLocked() is fine for just allocated pages that was not exposed
> > anywhere. After ->readpage() it's not the case and it's not safe to use
> > __SetPageLocked() for them.
> 
> In the current code, it's assumed it is not exposed, because a single read
> of a large page that does no readahead before the page is inserted into the
> cache means there are no external users of the page.

You've exposed the page to the filesystem once you call ->readpage().
It *may* track the page somehow after the call.

-- 
 Kirill A. Shutemov