How would you describe this situation:
Rolling one's eyes due to orgasm.
I checked most English dictionaries but most definitions are negative, e.g. due to frustration.
It's true that the expression "X rolled his/her eyes" is often associated with frustration or sarcasm.
A more neutral way of describing this expression is to say that "X's eyes rolled back", or "X's eyes rolled back in his/her head". This may refer to someone who is falling unconscious for whatever reason (e.g. it seems to show up fairly often in descriptions of "demonic possession"-type scenarios), but it's also used in erotic literature to describe the situation that you specified:
Her head and eyes rolled back, and she lost focus as the exquisite spasms erupted from her core and spread outward.
(p. 196, My Way Back to You, by Pamela Hearon, 2015)
Her eyes rolled back in her head as he watched her climax, and a low moan escaped her throat.
(p. 566, Dangerous Trust, by Cynthia Monet, 2009)
I'm not exactly sure what you're after, but the term I would use to describe this (as opposed to what my teenage daughter does pretty much any time I open my mouth) is:
La Petite Mort
or "the little death" in English. Wikipedia describes it as:
La petite mort is an expression which means "the brief loss or weakening of consciousness" and in modern usage refers specifically to "the sensation of orgasm as likened to death."
I recall coming across a photography project on the internet titled La Petite Mort that consisted of photos of people's faces as they reached orgasm, but the closest I could find now was a similar book by a Canadian photographer with the same name.