Re: [RFC PATCH] mm: shmem: allow split THP when truncating THP partially

[Yang Shi]

On 11/25/19 1:36 AM, Kirill A. Shutemov wrote:
> On Sat, Nov 23, 2019 at 09:05:32AM +0800, Yang Shi wrote:
> > Currently when truncating shmem file, if the range is partial of THP
> > (start or end is in the middle of THP), the pages actually will just get
> > cleared rather than being freed unless the range cover the whole THP.
> > Even though all the subpages are truncated (randomly or sequentially),
> > the THP may still be kept in page cache.  This might be fine for some
> > usecases which prefer preserving THP.
> >
> > But, when doing balloon inflation in QEMU, QEMU actually does hole punch
> > or MADV_DONTNEED in base page size granulairty if hugetlbfs is not used.
> > So, when using shmem THP as memory backend QEMU inflation actually doesn't
> > work as expected since it doesn't free memory.  But, the inflation
> > usecase really needs get the memory freed.  Anonymous THP will not get
> > freed right away too but it will be freed eventually when all subpages are
> > unmapped, but shmem THP would still stay in page cache.
> >
> > To protect the usecases which may prefer preserving THP, introduce a
> > new fallocate mode: FALLOC_FL_SPLIT_HPAGE, which means spltting THP is
> > preferred behavior if truncating partial THP.  This mode just makes
> > sense to tmpfs for the time being.
> We need to clarify interaction with khugepaged. This implementation
> doesn't do anything to prevent khugepaged from collapsing the range back
> to THP just after the split.

Yes, it doesn't. Will clarify this in the commit log.

> > @@ -976,8 +1022,31 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, loff_t lend,
> >   			}
> >   			unlock_page(page);
> >   		}
> > +rescan_split:
> >   		pagevec_remove_exceptionals(&pvec);
> >   		pagevec_release(&pvec);
> > +
> > +		if (split && PageTransCompound(page)) {
> > +			/* The THP may get freed under us */
> > +			if (!get_page_unless_zero(compound_head(page)))
> > +				goto rescan_out;
> > +
> > +			lock_page(page);
> > +
> > +			/*
> > +			 * The extra pins from page cache lookup have been
> > +			 * released by pagevec_release().
> > +			 */
> > +			if (!split_huge_page(page)) {
> > +				unlock_page(page);
> > +				put_page(page);
> > +				/* Re-look up page cache from current index */
> > +				goto again;
> > +			}
> > +			unlock_page(page);
> > +			put_page(page);
> > +		}
> > +rescan_out:
> >   		index++;
> >   	}
> Doing get_page_unless_zero() just after you've dropped the pin for the
> page looks very suboptimal.

If I don't drop the pins the THP can't be split. And, there might be 
more than one pins from find_get_entries() if I read the code correctly. 
For example, truncate 8K length in the middle of THP, the THP's refcount 
would get bumpped twice since two sub pages would be returned.

>