linux-kernel - Re: [RFC] RDMA/umem: pin_user_pages*() can temporarily fail due to migration glitches

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Date: Thu, 2 May 2024 19:44:58 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: John Hubbard <jhubbard@...dia.com>,
	David Hildenbrand <david@...hat.com>,
	Alistair Popple <apopple@...dia.com>,
	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>, linux-rdma@...r.kernel.org,
	linux-mm@...ck.org, Mike Marciniszyn <mike.marciniszyn@...el.com>,
	Leon Romanovsky <leon@...nel.org>,
	Artemy Kovalyov <artemyko@...dia.com>,
	Michael Guralnik <michaelgur@...dia.com>,
	Pak Markthub <pmarkthub@...dia.com>
Subject: Re: [RFC] RDMA/umem: pin_user_pages*() can temporarily fail due to
 migration glitches

On Thu, May 02, 2024 at 03:34:08PM -0300, Jason Gunthorpe wrote:
> IMHO pin_user_pages() should sleep and spin in an interruptable sleep

killable, not interruptible.  Otherwise SIGWINCH and SIGALRM can
result an early return.

> until we get all the migrations done. Not sure how hard it would be to
> add some kind of proper waiting event sleep?

ummmmm.  We have a "has waiters" bit in the folio.  So on every call to
folio_put(), we could check that bit and wake up any waiters.  I need to
think about that; right now, we only use it for unlock and end_writeback.
Making folio_put() heavier is, well, quite a lot of call-sites.