linux-kernel - why do we need vmalloc_sync

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Message-ID: <20150613185828.GA32376@redhat.com>
Date:	Sat, 13 Jun 2015 20:58:28 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Andy Lutomirski <luto@...capital.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Brian Gerst <brgerst@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Waiman Long <Waiman.Long@...com>
Subject: why do we need vmalloc_sync_all?

I didn't read v2 yet, but I'd like to ask a question.

Why do we need vmalloc_sync_all()?

It has a single caller, register_die_notifier() which calls it without
any explanation. IMO, this needs a comment at least.

I am not sure I understand the changelog in 101f12af correctly, but
at first glance vmalloc_sync_all() is no longer needed at least on x86,
do_page_fault() no longer does notify_die(DIE_PAGE_FAULT). And btw
DIE_PAGE_FAULT has no users. DIE_MNI too...

Perhaps we can simply kill it on x86?

As for other architectures I am not sure. arch/tile implements
vmalloc_sync_all() and uses notify_die() in do_page_fault().

And in any case register_die_notifier()->vmalloc_sync() looks strange.
If (say) arch/tile needs this to fix the problem with modules, perhaps
it should do vmalloc_sync_all() in do_init_module() paths?

Oleg.

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