linux-kernel - Re: [PATCH v6 18/18] arm64/mm: Automatically fold contpte mappings

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Date: Tue, 25 Jun 2024 14:06:22 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Ryan Roberts <ryan.roberts@....com>
Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>,
	Kefeng Wang <wangkefeng.wang@...wei.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>, Ard Biesheuvel <ardb@...nel.org>,
	Marc Zyngier <maz@...nel.org>, James Morse <james.morse@....com>,
	Andrey Ryabinin <ryabinin.a.a@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mark Rutland <mark.rutland@....com>,
	David Hildenbrand <david@...hat.com>,
	John Hubbard <jhubbard@...dia.com>, Zi Yan <ziy@...dia.com>,
	Barry Song <21cnbao@...il.com>,
	Alistair Popple <apopple@...dia.com>,
	Yang Shi <shy828301@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"Yin, Fengwei" <fengwei.yin@...el.com>,
	linux-arm-kernel@...ts.infradead.org, x86@...nel.org,
	linuxppc-dev@...ts.ozlabs.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 18/18] arm64/mm: Automatically fold contpte mappings

On Tue, Jun 25, 2024 at 01:41:02PM +0100, Ryan Roberts wrote:
> On 25/06/2024 13:37, Baolin Wang wrote:
> 
> [...]
> 
> >>> For other filesystems, like ext4, I did not found the logic to determin what
> >>> size of folio to allocate in writable mmap() path
> >>
> >> Yes I'd be keen to understand this to. When I was doing contpte, page cache
> >> would only allocate large folios for readahead. So that's why I wouldn't have
> > 
> > You mean non-large folios, right?
> 
> No I mean that at the time I wrote contpte, the policy was to allocate an
> order-0 folio for any writes that missed in the page cache, and allocate large
> folios only when doing readahead from storage into page cache. The test that is
> regressing is doing writes.

mmap() faults also use readahead.

filemap_fault():

        folio = filemap_get_folio(mapping, index);
        if (likely(!IS_ERR(folio))) {
                if (!(vmf->flags & FAULT_FLAG_TRIED))
                        fpin = do_async_mmap_readahead(vmf, folio);
which does:
        if (folio_test_readahead(folio)) {
                fpin = maybe_unlock_mmap_for_io(vmf, fpin);
                page_cache_async_ra(&ractl, folio, ra->ra_pages);

which has been there in one form or another since 2007 (3ea89ee86a82).