linux-kernel - Re: [PATCH 01/12] uprobes: split THPs before trying replace them

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Date:   Wed, 25 Jan 2017 08:55:22 -0800
From:   Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Hugh Dickins <hughd@...gle.com>,
        Rik van Riel <riel@...hat.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Oleg Nesterov <oleg@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 01/12] uprobes: split THPs before trying replace them

> > 
> > > For THPs page_check_address() always fails. It's better to split them
> > > first before trying to replace.
> > 
> > So what does this mean.  uprobes simply fails to work when trying to
> > place a probe into a THP memory region?
> 
> Looks like we can end up with endless retry loop in uprobe_write_opcode().
> 
> > How come nobody noticed (and reported) this when using the feature?
> 
> I guess it's not often used for anon memory.
> 

The first time the breakpoint is hit on a page, it replaces the text
page with anon page.  Now lets assume we insert breakpoints in all the
pages in a range. Here each page is individually replaced by a non THP
anonpage. (since we dont have bulk breakpoint insertion support,
breakpoint insertion happens one at a time). Now the only interesting
case may be when each of these replaced pages happen to be physically
contiguous so that THP kicks in to replace all of these pages with one
THP page. Can happen in practice?

Are there any other cases that I have missed?

-- 
Thanks and Regards
Srikar Dronamraju