Re: Calls to do_swap_page() from handle_pte_fault() on a system where swap is not configured

[Gregory Price]

On Wed, Dec 03, 2025 at 01:11:24PM +0530, Jayaraj Rajappan wrote:
> Hi,
> 
> On a system where swap is not configured, profiling using Linux "perf"
> tool shows that do_swap_page() gets called from handle_pte_fault().
> Kernel version is 5.14.0. HugePages are disabled on the system. Trying
> to understand what could cause do_swap_page() to be called when there
> is no swap configured on the system.
> 

all do_swap_page() call means is that the PTE is valid (something is
there) but not present (the present-bit is not set, or whatever that
particular architecture considers is present is not met)

static vm_fault_t handle_pte_fault(struct vm_fault *vmf)
{
...
        if (!vmf->pte)
                return do_pte_missing(vmf);

        if (!pte_present(vmf->orig_pte))
                return do_swap_page(vmf);
}


There are other non-swap swap-entries that are handled by the same code.
Transient migrations, device private, etc etc.

swap-entries are being renamed "softleaf" entries here shortly:
https://lore.kernel.org/linux-mm/cover.1762812360.git.lorenzo.stoakes@xxxxxxxxxx/

So it means "a software-entry is present" which could be any of:
enum softleaf_type {
	/* Fundamental types. */
	SOFTLEAF_NONE,
	SOFTLEAF_SWAP,
	/* Migration types. */
	SOFTLEAF_MIGRATION_READ,
	SOFTLEAF_MIGRATION_READ_EXCLUSIVE,
	SOFTLEAF_MIGRATION_WRITE,
	/* Device types. */
	SOFTLEAF_DEVICE_PRIVATE_READ,
	SOFTLEAF_DEVICE_PRIVATE_WRITE,
	SOFTLEAF_DEVICE_EXCLUSIVE,
	/* H/W posion types. */
	SOFTLEAF_HWPOISON,
	/* Marker types. */
	SOFTLEAF_MARKER,
};

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+Cc Lorenzo:

do_swap_page() is a stale name now probably?

I know there was a hold-off on changing actual swap code, but maybe
worth changing do_swap_page -> do_softleaf_page?

Not build or anything tested (sorry it's 3am insomnia time), basically just:

s/do_swap_page/do_softleaf_page
s/DO_SWAP_PAGE/DO_SOFTLEAF_PAGE

No clue what's up with the special sparc stuff.

~Gregory

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