Re: freespace.c modifies buffer without any locks

[Heikki Linnakangas]

On 29/10/2024 02:50, Andres Freund wrote:
Hi,
I just noticed that fsm_vacuum_page() modifies a buffer without even holding a
shared lock.  That quite obviously seems like a violation of the buffer
locking protocol:

        /*
         * Try to reset the next slot pointer. This encourages the use of
         * low-numbered pages, increasing the chances that a later vacuum can
         * truncate the relation.  We don't bother with a lock here, nor with
         * marking the page dirty if it wasn't already, since this is just a 
hint.
         */
        if (BufferPrepareToSetHintBits(buf))
        {
                ((FSMPage) PageGetContents(page))->fp_next_slot = 0;
                BufferFinishSetHintBits(buf);
        }


In the commit (15c121b3ed7) adding the current freespace code, there wasn't
even a comment remarking upon that oddity.  10 years later Tom added a
comment, in 2b1759e2675f.


I noticed this while adding a debug mode in which buffers are mprotected
PROT_NONE/PROT_READ/PROT_READ|PROT_WRITE depending on the buffer's state.


Is there any good reason to avoid a lock here? Compared to the cost of
exclusively locking buffers during RecordAndGetPageWithFreeSpace() the cost of
doing so during FreeSpaceMapVacuum*() seems small?

Agreed. This is a premature optimization, fsm_vacuum_page() should just 
take the lock.

Somewhat relatedly, but I don't think I understand why it's a good idea to
reset fp_next_slot to 0 in fsm_vacuum_page(). At least doing so
unconditionally.

Per the comment: "This encourages the use of low-numbered pages, 
increasing the chances that a later vacuum can truncate the relation".

Yes, the next GetPageWithFreeSpace() call will need to do a little more 
work to find the first page that actually has free space, if any. But 
that seems insignificant compared to vacuum.

When extending a relation, it seems we'll constantly reset the search back to
the start of the range, even though we pretty much know that there's no space
earlier in the relation - otherwise we'd not have extended.

That's a good point. Before commit a063baaced, relation extension used a 
separate UpdateFreeSpaceMap() function, which didn't reset fp_next_slot.

And when called from FreeSpaceMapVacuumRange() we'll reset fp_next_slot to
somewhere that wasn't actually vacuumed, afaict?

Yeah. In the context of actual VACUUM rather than relation extension, 
that seems fine; the next GetPageWithFreeSpace() call will fix it up 
quickly.

--
Heikki Linnakangas
Neon (https://neon.tech)