linux-kernel - Re: [PATCH v3] mm,thp: recheck each page before collapsing file THP

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Message-Id: <20191018181712.91dd9e9f9941642300e1b8d9@linux-foundation.org>
Date:   Fri, 18 Oct 2019 18:17:12 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Song Liu <songliubraving@...com>
Cc:     <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
        <matthew.wilcox@...cle.com>, <kernel-team@...com>,
        <william.kucharski@...cle.com>, <kirill.shutemov@...ux.intel.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH v3] mm,thp: recheck each page before collapsing file THP

On Fri, 18 Oct 2019 11:03:45 -0700 Song Liu <songliubraving@...com> wrote:

> In collapse_file(), after locking the page, it is necessary to recheck
> that the page is up-to-date. Add PageUptodate() check for both shmem THP
> and file THP.
> 
> Current khugepaged should not try to collapse dirty file THP, because it
> is limited to read only text. Add a PageDirty check and warning for file
> THP. This is added after page_mapping() check, because if the page is
> truncated, it might be dirty.

When fixing a bug, please always fully describe the end-user visible
effects of that bug.  This is vital information for people who are
considering the fix for backporting.

I'm suspecting that you've found a race condition which can trigger a
VM_BUG_ON_PAGE(), which is rather serious.  But that was just a wild
guess.  Please don't make us wildly guess :(

The old code looked rather alarming:

			} else if (!PageUptodate(page)) {
				xas_unlock_irq(&xas);
				wait_on_page_locked(page);
				if (!trylock_page(page)) {
					result = SCAN_PAGE_LOCK;
					goto xa_unlocked;
				}
				get_page(page);

We don't have a ref on that page.  After we've released the xarray lock
we have no business playing with *page at all, correct?