linux-kernel - Re: [PATCH v11 2/8] iov_iter: Add a function to extract a page list from an iterator

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Date:   Fri, 27 Jan 2023 01:10:39 +0100
From:   David Hildenbrand <david@...hat.com>
To:     David Howells <dhowells@...hat.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        Jens Axboe <axboe@...nel.dk>, Jan Kara <jack@...e.cz>,
        Jeff Layton <jlayton@...nel.org>,
        Jason Gunthorpe <jgg@...dia.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Christoph Hellwig <hch@....de>,
        John Hubbard <jhubbard@...dia.com>
Subject: Re: [PATCH v11 2/8] iov_iter: Add a function to extract a page list
 from an iterator

On 27.01.23 00:56, David Howells wrote:
> Al says that pinning a page (ie. FOLL_PIN) could cause a deadlock if a page is
> vmspliced into a pipe with the pipe holding a pin on it because pinned pages
> are removed from all page tables.  Is this actually the case?  I can't see
> offhand where in mm/gup.c it does this.

Pinning a page is mostly taking a "special" reference on the page, 
indicating to the system that the page maybe pinned. For an ordinary 
order-0 page, this is increasing the refcount by 1024 instead of 1.

In addition, we'll do some COW-unsharing magic depending on the page 
type (e.g., anon vs. fike-backed), and FOLL_LONGTERM. So if the page is 
mapped R/O only and we want to pin it R/O (!FOLL_WRITE), we might 
replace it in the page table by a different page via a fault 
(FAULT_FLAG_UNSHARE).

Last but not least, with FOLL_LONGTERM we will make sure to migrate the 
target page off of MIGRATE_MOVABLE/CMA memory where the unmovable page 
(while pinned) could otherwise cause trouble (e.g., blocking memory 
hotunplug). So again, we'd replace it in the page tale by a different 
page via a fault.

In all cases, the page won't be unmapped from the page table.

-- 
Thanks,

David / dhildenb