Re: [PATCH] 9p/virtio: restrict page pinning to user_backed_iter() iovec

[Matthew Wilcox <willy@infradead.org>]

On Wed, Dec 10, 2025 at 06:04:23AM +0900, Dominique Martinet via B4 Relay wrote:
> The problem is that iov_iter_get_pages_alloc2() apparently cannot be
> called on folios (as illustrated by the backtrace below), so limit what
> iov we can pin from !iov_iter_is_kvec() to user_backed_iter()
> 
> Full backtrace:
> ```
> [   31.395721][   T62] page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x102600
> [   31.395833][   T62] head: order:9 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0
> [   31.395915][   T62] flags: 0x2ffff800000040(head|node=0|zone=2|lastcpupid=0x1ffff)
> [   31.395976][   T62] page_type: f8(unknown)

This _isn't_ a folio.  It's a kmalloc allocation.  It's a very large
kmalloc allocation (something between 1024 * 1024 + 1 and 2048 * 1024
bytes inclusive).  You can _only_ use iov_iter_get_pages_alloc2() with
folios (or other struct pages that have a refcount, but I'm not sure how
many of those there really are; we're removing refcounts from various
types of pages)