From: Andrew Morton <akpm@linux-foundation.org>
To: Song Liu <songliubraving@fb.com>
Cc: <linux-kernel@vger.kernel.org>, <linux-mm@kvack.org>,
<matthew.wilcox@oracle.com>, <kernel-team@fb.com>,
<william.kucharski@oracle.com>, <kirill.shutemov@linux.intel.com>,
Johannes Weiner <hannes@cmpxchg.org>,
Hugh Dickins <hughd@google.com>
Subject: Re: [PATCH v3] mm,thp: recheck each page before collapsing file THP
Date: Fri, 18 Oct 2019 18:17:12 -0700 [thread overview]
Message-ID: <20191018181712.91dd9e9f9941642300e1b8d9@linux-foundation.org> (raw)
In-Reply-To: <20191018180345.4188310-1-songliubraving@fb.com>
On Fri, 18 Oct 2019 11:03:45 -0700 Song Liu <songliubraving@fb.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?
next prev parent reply other threads:[~2019-10-19 1:17 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-18 18:03 Song Liu
2019-10-19 1:17 ` Andrew Morton [this message]
2019-10-19 5:24 ` Song Liu
2019-10-21 18:03 ` Johannes Weiner
2019-11-06 4:58 ` Andrew Morton
2019-11-06 5:45 ` Song Liu
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